Part 3 Production System Design

7/30/2019 Part 3 Production System Design

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Part 3 Production System Design

3.1 Product and Service Design

Organizations that have well-designed products or services are more likely to realize their goalsthan those with poorly designed products or services.

Objectives of product and service design: profit, competitive, customer satisfaction, and quality.

Trends in product and service design:

profit,o low cost,o less material and less packaging.

customer satisfaction,o capabilities of production and delivery.

competitive,o reduced production time,o reduced introduction lead time.

quality,o environmental concern, including waste minimization, recycling parts, and

disposal of worn-out products,o user-friendly.

Other reasons for product and service design:

creating an image of being a market leader, customer complaints, accidents or injuries, excess warranty claims, and low demand.

Where to get design ideas?

Marketing, such as the use of focus groups, surveys, and analyses of buying patterns. Research and development, involving basic research, applied research, and development

from universities, research foundations, government agencies, and private enterprises.o Basic research has the objective of advancing the state of knowledge about a

subject without any near-term expectation of commercial applications.o Applied research has the objective of achieving applications.o Development converts the results of applied research into useful commercial

applications.o Benefits come from licensing and royalties of patents with potentially high cost.o Because product innovations made by US companies have ended up being

produced competitively by foreign companies with better processes, some


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3.1.1 Product Design Issues

Product Life Cycle

Incubationo Not familiaro Bugso Expectation of price drops


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Growtho Reliableo Affordableo Increased awareness

Maturityo

Few design changes Saturation Decline

Some products do not exhibit life cycles, e.g., wooden pencils, paper clips, knives, drinkingglasses, and similar items.

Strategies to prolong product life cycle:

reliability improvement, reduced cost and price,

product redesign, and packaging change.

Manufacturing Design

Design for manufacturing (DFM). Designs take into account the organizationscapabilities, when designing a product.

Design for recycling (DFR). Design facilitates the recovery of materials and componentsin used products for reuse.

Design for assembly (DFA). Design focuses on reducing the number of parts in a productand on assembly methods and sequence.

Design for dis-assembly (DFD). Design so that used products can be easily taken apart(using snap-fits wherever possible).

Robust design. Design that results in products or services that can function over a broadrange of conditions, and, eventually, results in higher level of customer satisfaction.

o Taguchis approach. Achieve major advance in product or service design fairlyquickly using a relatively small number of experiments. Using proper parameter

design, a classical design of combinations can be reduced to 32combinations to reach an optimal design.

Modular design. A form of standardization in which component parts are subdivided intomodules that are easily replaced or interchanged. It shares the pros and cons ofstandardization. Any failure is easy to diagnose and remedy but, if a part of a module

fails, the entire module must be scrapped. Concurrent engineering (simultaneously development). Bringing manufacturing design

and manufacturing personnel together early in the design phase to achieve smoothtransition from product design to production, and to decrease product development time.The group can be enlarged to include suppliers and marketing and purchasing personnel.

Why remanufacturing becomes popular?


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A remanufactured product can be sold for about 50 percent of the cost of the newproduct.

The remanufacturing process requires mostly unskilled and semi-skilled workers. Lawmakers increasingly require manufacturers to take back used products, because this

means fewer products end up in landfills and there is less depletion of natural resources

such as raw materials and fuel.

In concurrent engineering,

Knowledge of production capabilities helps the selection of suitable materials andprocesses.

The conflict in cost and quality can be greatly reduced. The product development process can be shortened with early procurement or design of

critical tooling. Serious product problems can be avoided due to early consideration of a design. Emphasis will focus on problem resolution instead of conflict resolution.

Existing boundaries between design and manufacturing can be difficult to overcome. Extra communication and flexibility are required to make concurrent engineering to

work.

Benefits of computer-aided design (CAD):

Increasing productivity of designers from 3 to 10 times. Supplying needed information through the creation of a part and manufacturing database. Allowing cost analysis and testing on a proposed design. Identifying the best design quickly among alterative designs.

3.1.2 Service Design Issues

In some cases, product design and service design go hand in hand. Managers must beknowledgeable about both in order to be able to manage effectively.


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Differences between service design and product design:

1. Service design focuses on intangible factors (e.g., peace of mind, ambiance) than doesproduct design.

2. Training process design and customer relations are particular important in service design,because services are created and delivered together and there is little time in finding andcorrecting errors before customers discover them.

3. Services can not be inventoried. This poses restriction on flexibility and makes capacitydesign very important.

4. Services are highly visible to customers and must be designed with that in mind.5. Some services have low barriers to entry and exit. This places additional pressures on

service design to be innovative and cost-effective.6. Location is often important to service design with convenience as a major factor.

Design guidelines:

Determine the nature and focus of the service and the target market. Determine customer requirements and expectations. Determine the degree of customer contact and customer involvement in the system. Have a single and unifying theme to help personnel to work together. Make sure the system has the capability to handle any expected variability in service

requirements. Make sure the system will be reliable and will provide consistently high quality. Design the system to be user-friendly.

Two key issues in service design are the degree of variation in service requirements, and thedegree of customer contact and customer involvement in the delivery system. These have animpact on the degree to which service can be standardized or must be customized. The lower thedegree of customer contact and service requirement variability, the more standardized the servicecan be (not true in general).


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Service blueprint: A method used in service design to describe and analyze a proposed service.

Blueprint design steps:

1. Establish boundaries and levels of details needed.2. Identify and describe the steps involved.3. Include major process steps in a flowchart.4. Identify potential failure points to minimize the chances of failures.5. Establish a timeframe for service execution.6. Analyze profitability.

3.1.3 A Product and Service Design ExampleQuality Function Deployment

(QFD)

Quality function deployment (QFD) is a structured approach for integrating the voice ofcustomers into the product or service development process. The purpose is to ensure that

customer requirements are factored into every aspect of the production or delivery process. Oncethe requirements are known, they must be translated into technical terms related to the product orservice.

The structure of QFD is based on a set of matrices. The main matrix relates customerrequirements (what) and their corresponding technical requirements (how).


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Additional features are usually added to the basic matrix to broaden the scope of analysis.Typical additional features include importance weightings and competitive evaluations.


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A correlation matrix is usually constructed for technical requirements; this can reveal conflictingtechnical requirements. The resulting matrix is often referred to as the house of quality, becauseof its house-like appearance.

An analysis using the format is shown below for a commercial printer (customer) and the

company that suppliers papers.


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3.1.4 Reliability


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Reliability is a measure of the ability of a product, part, or a system to perform its intendedfunction under a prescribed set of conditions. In effect, reliability is a probability. A reliability of0.985 implies 15 failures per 1,000 parts of trials.

Probability is used in the following two ways:

1. The probability that the product or the system will function when activated. This is oftenused, when a system is used for a relatively few number of times.

2. The probability that the product or the system will function for a given length of time.This is often used in product warranty.

System Reliability

Determining the probability, when the product or system consists of a number of independentcomponents requires the use of probability rules for independent events, i.e., events having norelation to the occurrence or nonoccurrence of each other.

Rule 1. If two or more events are independent and success is defined as the probability that allof the events occur, the probability of success is

,

where

: event ,

: success probability of event , and

: total number of events.

Rule 2. For three independent events, if two of the three events are backup events and successis defined as the probability that at least one of them occurs, the probability of success is

.

The second way of looking at reliability considers the incorporation of a time dimension. The

following figure shows a products failure rate over time. It is sometimes referred to as a bathtubcurve.


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Mean Time Between Failures

The mean time between failures (MTBF) in the infant mortality phase can be modeled by anegative exponential distribution (cumulative distribution function).

,

where

= natural logarithm, 2.7183,

= length of service before failure, and

MTBF=mean time between failures.

The probability is equal to the area under the curve for . Observe that, as the specifiedlength of service increases, the area under the curve to the right decreases. You can obtain theprobability using a table of exponential values with a mean having a value of MTBF. Theprobability that failure will occur before Tis

.


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Reliability Under Normal Distribution

Product failure due to wear-out can be modeled by a normal distribution with a parameter ,where is a standardize value computed using the formula

.

To obtain a probability that service life will not exceed , compute and refer to the normaldistribution table. Next, to find the reliability for time , subtract this probability from 100percent.

To obtain the value of that will provide a given probability, locate the nearest probabilityunder the curve to the left in the table. Then, use the corresponding in the proceeding formulaand solve for .


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Availability

Availability measures the fraction of time a piece of equipment is expected to be operational.Availability can range from 0 to 1. Availability is a function of the mean time between failures(MTBF) and the mean time to repair (MTR).

.

The availability formula reveals two implications: one is that availability increases as the MTBFincreases; the other is that the availability increases as the MTR decreases.

Some design options enhance reparability, e.g., laser printers are designed with printer cartridgesthan can easily be replaced.

Companies that can offer equipment with a high availability factor have a competitive advantage

over companies that offer equipment with low availability values.

Reliability Summary

The importance of reliability is understood by its use by perspective buyers in comparingalternatives and by seller as one determinant of price. Reliability also can have an impact onrepeat sales, reflect on theproducts image and create legal implication.

The term failure is used to describe a situation in which an item does not perform as intended.Reliability is always specified with respect to certain conditions, called normal operatingconditions. Failure of users to heed these conditions often results premature failure of parts or

complete systems.

Directions of improving reliability:

Improve component design. Improve production and/or assemble techniques. Improve testing. Use backup components. Improve preventive maintenance procedures. Improve user education. Improve system design.

The reliability improvement needed depends on the potential benefit of the improvements and onthe cost of those improvements. In the long term, efforts to improve reliability and reduce costswill lead to high levels of reliability.

3.1.5 Operations Strategy in Product and Service Design

Invest more in R&D.


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Shift some emphasis away from short-term performance to long-term performance. Work toward continual (albeit gradual) improvements instead of using a big bang

approach. Shorted the product development cycle.

3.2 Process Selection

The process selection of an organization is determined by the organizations process strategy

below.

Make or buy decision: The extent to which the organization will produce goods orprovide services in-house as opposed to relying on outside organizations to produce orprovide them.

Capital intensity: The mix of equipment and labor that will be used by the organization. Process flexibility: The degree to which the system can be adjusted to changes in

processing requirements due to such factors as changes in product or service design,

changes in volume processed, and changes in technology.

3.2.1 Make or Buy

A number of factors are considered in a make or buy decision.

1. Availability capacity, equipment, skill, expertise, and time,2. Quality,3. Steady v.s. fluctuated demand., and4. Cost.

If an organization decides to perform some or all of the processing, then the issue of processselection becomes important.

3.2.2 Type of Operation

The degree of standardization and the volume of outputof a product or a service influence theway production is organized.

Continuous processing. Semi-continuous (repetitive) processing. Intermittent processing.

o Batch processing.o Job shop processing.

Project. Automation.


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For products or services with life cycles, a manager must know when to shift from one type ofprocess to the next.

Projectis a set of activities directed toward a unique goal.


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Automation is machinery that has sensing and control devices that enable it to operateautomatically. A key question in automation is whether to automate and how much to automate.

Stable quality and cost. Expensive to implement.

Possible adverse effect on moral and productivity.

Automation can range from factories that are completely automated to a single automatedoperation.

3.2.3 Automation

Generally speaking, there are three kinds of automation: fixed, programmable, and flexible.

Fixed automation (Detroit-type automation).o Use high-cost and specialized equipment for a fixed sequence of operations.o Primary advantages are low cost and high volume.o Primary limitations are minimal variety and high cost for making major changes

in products or processes. Programmable automation.

o Computed-aided manufacturing (CAM). Numerically controlled (NC) machines. The main limitations of NC

machines are the higher skill levels needed to program the machines andtheir inability to detect tool wear and material variation.

Computerized numerically controlled (CNC) machines, NC machines withtheir own computers.

Directed numerical control (DNC), a cluster of NC machines controlled bya computer.

Robots, powered pneumatically (air-driven), hydraulically (fluids underpressure), or electronically.

Flexible automation.o Manufacturing cell (MC), one or a small number of NC machines producing a

family of similar products.o Flexible manufacturing system (FMS), a group of machines which can be

reprogrammed to produce a variety of similar products.o Computer-integrated manufacturing (CIM), a system that uses an integrated

computer system to link a broad range of manufacturing activities, includingengineering design, flexible manufacturing systems, and production planning and

control.

Fixed automation is the most rigid of the three types. It uses high-cost and specializedequipments for a fixed sequence of operations. Low cost and high volume are its primaryadvantages; minimal variety and the high cost of making major changes in either production orprocess are its primary limitation.


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Programmable automation is at the opposite end of the spectrum. It involves the use of high-costand general-purpose equipments controlled by a computer program. The program provides boththe sequence of operations and specific details about each operation. Changing the process iseasy (or difficult) as changing the computer program, and there is downtown while programchanges are being made. This type of automation has the capability of economically producing a

fairly wide variety of low-volume products in small batches. Numerical controlled (NC)machines and some robots are applications of programmable automation.

Computer-aided manufacturing (CAM) refers to the use of computers in process control, rangingfrom robots to automated quality control.

Flexible automation evolved from programmable automation. It uses equipment that is morecustomized than that of programmable automation. A key difference between the two is thatflexible automation requires significantly less changeover time. This permits almost continuousoperation of equipment and product variety without the need to produce in batches.

Machines involved in FMS include supervisory computer control, automatic material handling,and robots or other automated processing equipments. FMS appeals to managers who hope toachieve both the flexibility of job shop processing and the productivity of repetitive processingsystems. Disadvantages include

Handling only a relatively narrow range of part variety, Requiring long planning and development, and Representing a sizable chunk of technology (not gradual implementation).

The overall goal of using CIM is to link various parts of an organization to achieve rapidresponse to customer orders and/or product changes to allow rapid production and to reduce

indirect labor costs. A shining example can be found at Allen-Bradleys CIM process inMilwaukee, Wisconsin.

3.3 Capacity Planning

Capacity refers to an upper limit or ceiling on the load that an operating unit can handle. Theoperating unit may be a plant, department, machine, store, or worker. Capacity enables managersto quantify production capability in terms of inputs or outputs.

Capacity planning is the most important design decision among others. The reasons are

1. Capacity decisions have a real impact on the ability of the organization to meet futuredemand for products and services, and, therefore, can affect competitiveness.

2. Capacity decisions affect operating costs and capacity is usually a major determinant ofinitial cost.

3. Capacity decisions often involve long-term commitment of resources.Three basic questions in capacity planning are


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1. What capacity is needed?2. How much is needed?3. When is needed?

Capacity planning can be an on-going process. Generally, the factors that influence the

frequency of capacity planning are the stability of demand, the rate of technological change inequipment and product design, and competitive factors.

Two useful definitions of capacity are

1. Design capacity: the maximum output that can possibly be attained.2. Effective capacity: the maximum possible output given a product mix, scheduling

difficulty, machine maintenance, quality factors, and so on.

Design capacity is the maximum rate of output achieved under ideal conditions. Effectivecapacity is usually less than design capacity (it cannot exceed design capacity). Actual output

cannot exceed effective capacity.

Two measures of system effectiveness by using the two definitions of capacity are

, and

.

It is common for managers to focus exclusively on efficiency, but in many instances, thisemphasis can be misleading. This happens when effective capacity is low compared with designcapacity.

The real key to improving capacity utilization is to increase effective capacity by correctingquality problems, maintaining equipment in operating condition, fully training employees, andfully utilizing bottleneck equipment.

It is important to recognize that the benefits of high utilization are only realized in instanceswhere there is demand for the output. Another disadvantage of high utilization is that operatingcosts may increase because of increasing waiting time due to bottleneck conditions.

Determinants of effective capacity:


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Capacity planning decisions involve both long-term and short-term considerations. Wedetermine long-term capacity needs by forecasting demand over a time horizon and then

converting those forecasts into capacity requirements. Short-term capacity needs are lessconcerned with cycles or trends than with seasonal variations and other variations from average.When time intervals are too short to have seasonal variations in demand, the analysis can oftendescribe the variations by probability distributions.

3.3.1 Decision Making in Capacity Planning

Typical capacity planning considerations include the following items.


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Design flexibility into systems for new products, new demand, or phase-out products. Take a big picture. When adding rooms in a motel, one should also take into account

probable increased demands for parking, food, entertainment, and housekeeping. Prepare to deal with capacity chunks. Capacity increases are often acquired in fairly large

chunks rather than smooth increments.

Dynamic capacity planning generally makes a system to alternate between under-utilization and over-utilization. Unfortunately, no simple solutions exist for the problem.Possible options are

o to identify products or services that have complementary demand patterns,

o to use overtime work,o

to subcontract some work, ando to replenish inventory during low-demand period.

Identify the optimal operating level. For low level of output, fixed equipment cost isshared by very few units. As output is increased, there are more units to absorb the fixedcost. However, beyond a certain point, unit cost starts to rise due to increasing load to thefacility.


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Both optimal operating rate and the amount of the minimum cost tend to be a function ofthe general capacity of the operating unit. As the general capacity of a plant increases, theoptimal output increases and the minimum cost for the optimal rate decreases.

Once alternatives of capacity planning are developed, items to evaluate the alternatives are

economical considerations, including costs of development, operating, and maintenance, availability (when and how soon will they be available), public opinions, environmental concerns, employee relocation, and cost-volume and financial analysis.

3.3.2 Cost-Volume Analysis

Cost-volume analysis is one of the common methods used in alternative evaluation in capacityplanning. Variables used in the analysis are provided below.


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Assumptions of cost-volume analysis are:

One product is involved. Everything produced can be sold. The variable cost per unit is the same regardless of the volume. Fixed costs do not change with volume changes, or they are step changes. The revenue per unit is the same regardless of volume.

In cost-volume analysis, the total cost (TC) is the sum of the fixed cost (FC) and the variable costper unit (VC) times output volume (Q).


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Total revenue (TR) is the revenue per unit (R) times the output volume (Q).


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The profit (P) is the difference the total revenue (TR) and the total cost (TC).


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The volume at which the total cost (TC) and the total revenue (TR) are equal, i.e., P=0, isreferred to as the break-even point (BEP). When the output volume is less than the break-even

point, there is a loss; when the output volume is greater than the break-even point, there is aprofit. The greater the deviation from this point, the greater the profit or loss.

The required quantity (Q) needed to generate a specified profit (P) is

.

Using this formula, the break-even quantity ( ) at P=0 is

.

Capacity alternatives may involve step costs. For example, a firm may the option of purchasingone, two, or three machines, with each additional machine increasing the fixed cost.


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This implies that multiple break-even quantities may occur, possibly one for each range. A

manager must consider projected annual demand (volume) relative to the multiple break-evenpoints and choose the most appropriate number of machines.

Other capacity planning analyses include financial analysis, decision theory, and waiting lineanalysis (details are not covered here).

3.3.3 Linear Programming

Linear programming is a powerful quantitative tool used by operations managers and othermanagers to obtain optimal solutions to problems that involve restrictions or limitations.

Linear Programming Models

A linear programming model has the following four components:

1. decision variables, choices available to decision makers,2. objective, a mathematical expression used to determine the total profit/cost for a given

solution,3. constraints, limitations of alternatives available to decision makers, and4. parameters, coefficients of the objective and the decision variables.

subject to


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where

A linear programming model has the following assumptions:

1. Linearity: linear objective function and constraints.2. Divisibility: decision variables have non-integer values.3. Certainty: parameters are known and constants.4. Non-negativity: values of decision variables are always greater than or equal to zeros.

Some nonlinear constraints can be converted into linear constraints to formulate the desiredlinear programming model.

Graphical Linear Programming

Graphical linear programming is a method for finding optimal solutions to two-variableproblems.

General steps of graphical linear programming:

1. Formulate a linear programming model by identifying its objective functions andconstraints.

2. Plot the constraints.3. Identify feasible solution space.4. Plot objective function.5. Determine the optimal solution.

Example: Find the optimal product mix by maximizing the total profit using the followingproduction data.

Type 1 Type 2

Profit per unit $60 $50Assembly time per unit 4 hours 10 hours

Inspection time per unit 2 hours 1 hour

Storage space per unit 3 cubic feet 3 cubic feet


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Resource Amount Available

Assembly time 100 hours

Inspection time 22 hours

Storage space 39 cubic feet

Linear programming model:

subject to

where

.

Plotting constraints:


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Identifying the feasible solution space:

Plotting objective function:


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Determining the optimal solution:

Optimal solution: .

Solution Properties of Linear Programming Models

A constraint is redundant, if its removal would not alter the feasible solution space.


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For some linear programming problems with two variables, it is possible to find the optimalsolution of the problems by using its finite number of corner points.

A two-variable linear programming problem has multiple optimal solutions, if two of its corner

points are optimal solutions of the problem. Its objective function is parallel to one of itsconstraints.

For some two-variable linear programming problems in the following format, its object functionhas an optimal solution closest to the origin of its feasible solution space.

subject to

where


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If a constraint forms the optimal corner point of the feasible solution space, it is called a binding

constraint. If the constraint could be relaxed, an improved solution would be possible.

For constraints, if the right side is greater than the left side of the constraints, we say there isslack. For constraints, if the left side is greater than the right side of the constraints, there issurplus.

The Simplex Method

The simplex method is a general-purpose linear programming algorithm widely used to solvelarge-scale problems. In practice, computers are used to solve these problems using the method.

Steps of using Excel Solver:

1. 1.Enter problem in a worksheet.


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2. Click Tools>Solver from the Tools menu. From the popped up window, enter targetcell (objective function), objective (max/min), and constraints. In the options button,click Assume Linear Model. Then, click Solve button to solve the problem.

3. If there is any error, correct errors and solve the problem again by clicking the Solvebutton. Otherwise, highlight both Answer and Sensitivity in the Report box and clickOK.

4. Solver provides the optimal values of decision variables and the objective function inthe worksheet.


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Sensitivity Analysis

Sensitivity analysis is a means of assessing the impact of potential changes to the parameters of alinear programming model. There are three types of potential changes:

1. Objective function coefficients. In a graphical solution, this means a possible change toanother corner point of the feasible solution space. There is generally a range of values,i.e., range of optimality, for which the optimal values of the decision variables will notchange. However, the optimal value of the objective function will change. In ExcelSolver, the range of optimality is provided by the Changing Cells section of the output.


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In some problems, one or more decision variables may be nonbasic (i.e., have an optimalvalue of 0). In such instances, unless the value of that variables objective function

coefficient increases by more than its reduced cost, it wouldnt come into solution (i.e.,become a basic variable). Hence, the range of optimality (sometimes referred to as therange of significance) for a nonbasic variable is from negative infinity to the sum of itscurrent value and its reduced cost.

For multiple changes to objective function coefficients, divide each coefficients changeby the allowable change in the same direction. Treat all resulting fractions as positive.Sum the fractions. If the sum does not exceed 1, then multiple changes are within therange of optimality.

2. Right-hand values of constraints. Each constraint has a corresponding shadow price,which is a marginal value that indicates the amount by which the value of the objectivefunction would change, if there were a one-unit change in the right-hand-side value of theconstraint.

The shadow price remains a constant over a limited range called the range of feasibility.Within the range of feasibility, multiplying the amount of change in the right-hand-sidevalue of a constraint by the constraints shadow price willindicate the changes impacton the optimal value of the objective function.

If a constraint is nonbonding, its shadow price is zero and the change to its right-hand-side is

bounded by its slack/surplus value.

In Excel Solver, the Constraints section in the output shows the shadow price and the range offeasibility of each constraint.

If there are changes to more than one constraints right-hand-side value, analyze these in thesame way as multiple changes to objective function coefficients.


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If a change goes beyond the range of feasibility, it will be necessary to recomputed the solution.

3. Constraint coefficients (not covered).3.4 Facility Layout

Layout refers to the configuration of departments, work centers, and equipment with particularemphasis on movement of work (customers and materials) through the system.

The need for layout decisions includes

Inefficient operations (e.g., high cost, bottlenects).

Accidents or safety hazards. Changes in the design of products or services. Introduction of new products or services. Changes in the volume of output of mix of outputs. Changes in methods or equipment. Changes in environmental or other legal requirements. Moral problems (e.g., lack of face-to-face contact).

3.4.1 Basic Layouts

The three basic types of layouts are product, process, and fixed-position.

Product Layouts

Product layouts are most conductive to repetitive processing. They use standardized processingoperations to achieve a smooth and rapid flow of large volumes of goods and customers througha system.


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The main advantages of product layouts are: High output rate, low unit cost, reduced trainingcost and time, low material-handling cost, high utilization of labor and equipment, establishedrouting and scheduling, and routine accounting, purchasing, and inventory control.

The primary disadvantages of product layouts include: dull and repetitive jobs, poor outputquality due to little interest in maintenance, susceptible to shutdowns, preventive maintenance,and impractical incentive to individuals.

U-shape layouts are more compact, flexible in work assignment, and teamwork-oriented.

Process Layouts

Process layouts are used for intermittent processing. The layouts feature departments or otherfunctional groupings in which similar kinds of activities are performed.


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The advantages of process layouts include:

The ability to handle a variety of processing requirements. Less equipment failures and dependency among equipments. Low maintenance cost and easy to maintain due to general-purpose-equipment. Possibility of individual incentive.

The disadvantages of process layouts include:

High in-process inventory cost, if batch processing is used. Frequent routing and scheduling. Low equipment utilization. Variable-path material-handling equipments (forklift trucks, jeeps, or tote boxes). Slow and inefficient material handling, unit material handling cost is high. High supervisory cost due to job complexity. Unit production cost is higher than the one in product layouts. Less routine accounting, inventory control, and purchasing. Skilled or semi-skilled workers.

Fixed-Position Layouts

Fixed-position layouts are used, when the item being worked on remains stationary, and workers,materials, and equipment are moved as needed. Fixed-point layouts are used in large

construction projects due to compelling reasons bringing workers, materials, and equipment totheproducts location instead of the other way around.


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Lack of storage space can present significant problems. The span of control is narrow. Special efforts are needed to coordinate activities.

Administrative burden is high. Material handling resembles process-type, variable-length, general-purpose equipment.

Hybrid Layouts

The three basic layout types are ideal models, which may be altered to satisfy the needs of aparticular situation.

For instance, supermarket layouts are essentially process layouts, and yet we find most use fixed-path material-handling devices in the stock room and belt-type conveyors at the cash registers.

Process layouts and product layouts represent two ends of a continuum from small batches tocontinuous production. Process layouts are conductive to the production of a wider range ofproducts or services than product layouts, which is desirable from a customer standpoint wherecustomized products are often in demand. However, process layouts tend to be less efficient andhave higher unit production costs than product layouts.

Ideally, a system is flexible and yet efficient with low unit production costs.

Cellular Layouts

Cellular layouts group machines into what is referred to as a cell. Groupings are determined by

the operations needed to perform work for a set of similar items, orpart families, that requiresimilar processing. The cells become, in effect, miniature versions of product layouts.


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There are numerous benefits of cellular manufacturing, including

fast processing time, less material handling, enhance the productivity of product design and process design, less work-in-process inventory, and reduced setup time.

Effective cellular manufacturing must have groups of identified items with similar processingcharacteristics. The grouping process is known as group technology (GT) and involvesidentifying items with similarities in either design characteristics or manufacturingcharacteristics, and grouping them into part families.

Design characteristics include size, shape, and function; manufacturing or processingcharacteristics involve the type and sequence of operations required. In general, design familiesmay be different from processing families.

Three primary methods commonly involved in group technology are


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Visual inspection (least accurate but least cost),

Examination of design and production data (commonly used), and

Production flow analysis.

Flexible manufacturing systems (FMSs) are more fully automated versions of cellularmanufacturing.

Other Layouts

Warehouse and storage layouts. Layout properties are demand, demand correlation, productexpiration, inventory counting, traffic, storage size, aisle size, and cargo loading/unloading.

Retail layouts. Layout properties are customer traffic patterns and traffic flow, standard layoutsacross national stores.

Office layouts. Layout properties are the flow of paperwork being replaced by electroniccommunication, and projecting an image of openness like low-rise partitions and glass walls.

3.4.2 Layout Design

Product Layout Design - Line/Load Balancing

Many of the benefits of a product layout relate to the ability to divide required work into a seriesof elemental tasks that can be performed quickly and routinely by low-skilled workers orspecialized equipment. The durations of these elemental tasks typically range from a few secondsto 15 minutes or more.

The process of deciding how to assign tasks to workstations is referred to as line balancing. Thegoal of line balancing is to obtain task groupings that represent approximately equal timerequirements. This minimizes the idle time along the line and results in a high utilization of laborand equipment.

The major obstacle to attaining a perfectly balanced line is the difficulty of forming task bundlesthat have the same duration. Common causes are

1. Equipment requirements are different.2. Activities are not compatible.3. A required technological sequence may prohibit otherwise desirable task combinations.

Assuming a workstation has a worker, the primary determinant in line balancing design is cycle

time, the maximum time allowed at each workstation to perform assigned tasks before the workmoves on.

Let the task times and precedence relationships be as shown in the following diagram.


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The task time govern the range of possible cycle times. The minimum cycle time is equal to thelongesttask time (1.0 minute), and the maximum cycle time is equal to the sum of the task times(0.1 + 0.7 + 1.0 + 0.5 + 0.2 = 2.5 minutes).

The minimum and maximum cycle times are important, because they establish the potentialrange of output for the line, which we can compute using the following formula:

,

where

OT= operating time per day, andCT= cycle time.

With an operation of 480 minutes and the minimum and maximum cycle times, the outputselected for the line must fall in the range of 192 units per day to 480 units per day.

In general, the cycle time is determined by the desired output. That is,

,

where

D= desired output rate.

Then, a theoretical minimum number of stations necessary to provide a special rate of output is

,

where

= theoretical minimum number of stations,

CT= cycle time from desired output rate, and

= sum of task times.

Another required tool in line balancing is a precedence diagram. It visually portrays the tasksthat are to be performed along with the sequential requirements, that is, the orderin which tasksmust be performed.


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Finally, the assignment of tasks to workstations may use heuristic rules for the decision, whichprovide good and sometimes optimal sets of assignments. Common line-balancing heuristic rulesare

1. Assign tasks in order of the most number of following tasks.2. Assign tasks in order of greatest positional weight. Positional weight is the sum of each

tasks time and the times of all following tasks (Example 2 on pp.284).

To clarify the terminology,following tasks are all tasks that you would encounter by followingall paths from the task in question through the precedence diagram. Preceding tasks are all tasksyou would encounter by tracing all paths backwardfrom the task in question. In the precedencediagram below, tasks b, d, e, and f are followers of task a. Tasks a, b, and c are preceding tasksfor e.

Thepositional weightfor a task is the sum of task times for itself and all its following tasks.

The objective of line balancing is to minimize the idle time for the line subject to technologicaland output constraints.

Technological constraints can result from the precedence or ordering relationships among thetasks. The constraints may also result from two tasks being incompatible. Output constraints, onthe other hand, determine the maximum amount of work that a manager can assign to eachworkstation, and this determines whether an eligible task will fit at a workstation or not.

Following are general procedures used in line balancing.


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As a result, the assignment of those tasks into three workstations using a cycle time of 1.0 minuteis provided below.

Common effectiveness measures of line balancing are

1. Idle time percentage (balance delay).

.

2. Efficiency.

.

The minimum number of workstations needed is a function of the desired output rate; and,therefore, the cycle time. A lower rate of output (and, hence, a longer cycle time) may result in aneed for fewer workstations. The manager must consider whether the potential savings realizedby having fewer workstations would be greater than the decrease in profit resulting fromproducing fewer units.

In practice, task completion time will be variable and lines are rarely balanced whenever humansare involved. However, this is not entirely bad. Some unbalance means that slack exists at points

along the line, which can reduce the impact of brief stoppage at some workstations. Also,workstations that have slack can be used for new workers who may not be up to speed.

Other line balancing approaches include parallel workstations, dynamic line-balancing (cross-training), and mixed mode line. In the mixed mode line, a line is designed to handle more thanone product. The products have to be fairly similar to each other so the tasks involved would bemuch the same for the products.

Process Layout Design

Process layout design is a problem of assigning work centers to pre-configured locations. Layoutdesign can be influenced by the compatibility of tasks/centers and external factors. Externalfactors include the location of entrances, loading docks, elevators, windows, and areas ofreinforced flooring. Also important are noise levels, safety, and the size and locations ofrestrooms.

A major obstacle to finding the most efficient layout of work centers/tasks is the large number ofpossible assignments. Often planners must reply on heuristic rules to guide trial-and-error effortsfor a satisfactory solution to each problem.


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One of the major objectives in process layout is to minimize transportation cost, distance, ortime. Other concerns include initial costs in setting up the layout, expected operating costs, theamount of effective capacity created, and the ease of modifying the system like the costs ofrelocating any work center. Note that multilevel structures pose special problems for layoutplanners.

Common from-to charts help the design of process layouts. The charts should include thedistance between locations,

and the workflow between work centers.

A heuristic rule for minimizing the transportation costs can be assigning work centers with thegreatest work flow first to locations that are closest to each other.

Although the preceding approach is widely used, it suffers from the limitation of focusing ononly one objective, and many situations involve multiple criteria. Richard Muther and JohnWheeler developed a more general approach to the problem in 1962. The approach allows forsubjective input from analysis and managers to indicate the relative importance of eachcombination of work center pairs. The information is then summarized in the grid below.


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In practice, the letters on the grid are often accompanied by numbers that indicate the reason foreach assignment. Major limitations relate to the use of subjective inputs in general which may beimprecise and unreliable.

3.5 Design of Work Systems

Work design involves job design, work measurement and the establishment of time standards,and worker compensation. Work design is one of the oldest aspects of operations management.

It is important for management to make design of work systems a key element of its operationsstrategy. In spite of the major advances in computers and manufacturing technology, people arestill the heart of business; they can make or break it regardless of the technology used.Technology is important, of course, but technology alone is not enough.

People work for a variety of reasons. Economic necessity is among the most important, butbeyond that, people work for socialization, to give meaning and purpose to their lives, for status,for personal growth, and for other reasons. Theses motivations can play an important role in thelives of workers, and management should accord them serious consideration in the design ofwork systems.

3.5.1 Job Design

Job design involves specifying the content and methods of jobs. In general, the goal of job designis to create a work system that is productive and efficient. Job designers are concerned with whowill do a job, how the job will be done, and where the job will be done. To be successful, job

design must be:

1. Carried out by experienced personnel who have the necessary training and background.2. Consistent with the goals of the organization.3. In written form.4. Understood and agreed to by both management and employees.5. Workers and managers alike should be consulted in order to take advantage of their

knowledge and to keep them informed.


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Current practice in job design contains elements of two basic schools of thought. One might becalled the efficiencyschool, a refinement of Federick Winslow Taylors scientific managementconcepts, because it emphasizes a systemic and logical approach to job design; the other is calledthe behavioral school, because it emphasizes satisfaction of wants and needs.

The behavioral view received a shot in the arm in the 1970s when Work in America was released.Two points were of special interest to job designers. One was that many workers felt that theirjobs were not interesting; the other was that workers wanted more control over their jobs. Thecentral issue seemed to be the degree of specialization associated with jobs: high specializationappeared to generate the most dissatisfaction. It is noteworthy that specialization is a primaryissue of disagreement between the efficiency and behavioral approaches.

The term specialization describes jobs that have a very narrow scope. The following tableprovides major advantages and disadvantages of specialization in business.

Job Enlargement, Rotation, and Enrichment

In an effort to make jobs more interesting and meaningful, job designers frequently consider jobenlargement, job rotation, job enrichment, and increased use of mechanization.

Job enlargement means giving a worker a larger portion of the total task. This constituteshorizontal loading --- the additional work is on the same level of skill and responsibility as theoriginal job. The goal is to make the job more interesting by increasing the variety of skillsrequired and by providing the worker with a more recognizable contribution to the overalloutput. For example, a production workers job might be expanded so that he or she is

responsible for a sequence of activities instead of only one activity.

Job rotation means having workers periodically exchange jobs. A firm can use this approach toavoid having one or a few employees stuck in monotonous jobs. It works best when workers canbe transferred to more interesting jobs; there is little advantage in having workers exchange oneboring job for another. Job rotation allows workers to broaden their learning experience andenables them to fill in for others in the event of sickness or absenteeism.


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Job enrichment involves an increase in the level of responsibility for planning and contributiontasks. It is sometimes referred to as vertical loading. An example of this is to have stock clerks insupermarkets handle reordering of goods, thus increasing their responsibilities. The jobenrichment approach focuses on the motivating potential of worker satisfaction.

In addition to the aforementioned approaches, organizations are also experimenting with choicesof locations (e.g., medium-sized cities or campus-like settings), flexible work hours, and teams.

Teams

Self-directed teams, sometimes referred to as self-managed teams, are designed to achieve ahigher level of teamwork and employee involvement. The underlying concept is that theworkers, who are close to the process and have the best knowledge of it, are better suited thanmanagement to make the most effective changes to improve the process. Moreover, because theyhave a vested interest and personal involvement in the changes, they tend to work harder toensure that the desired results are achieved than they would, if management had implemented the

changes. For these teams to function properly, team members must be trained in quality, processimprovement, and teamwork.

Self-directed teams have the following benefits.

Fewer managers are required.

Teams can provide improved responsiveness to problems.

Teams have a personal stake in making the process work.

Teams require less time to implement improvement.

Teams lead to higher quality, higher productivity, and greater worker satisfaction.

Greater worker satisfaction leads to less turnover and absenteeism, resulting in low coststo train new workers and less need to fill in for absent employees.

However, managers, particularly middle managers, often feel threatened as teams assume moreof the traditional functions of managers.

Methods Analysis

Methods analysis focuses on how a job is done. Methods analysis is done both for existing jobsand new jobs. The need for methods analysis can come from a number of different sources:

1. Changes in tools and equipment.2. Changes in product design or new products.3. Changes in materials or procedures.

4. Government regulations or contractual agreements.5. Other factors (e.g., accidents, quality problems).

The basic procedure in methods analysis is:

1. Identify the operation to be studied, and gather all pertinent facts about tools, equipment,materials, and so on. General guidelines of selecting an operation are:

a. Have a high labor content.b. Are done frequently.


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c. Are unsafe, tiring, unpleasant, and/or noisy.d. Are designated as problems (e.g., quality problems, scheduling bottlenecks).

2. For existing jobs, discuss the job with the operator and supervisor to get their input.3. Study and document the present method of an existing job using process charts.

Analyzing and improving methods is facilitated by the use of various charts such asflow

process charts and work-machine charts. For new jobs, develop charts based oninformation about the activities involved.4. Analyze the job.5. Proposed new methods.6. Install the new methods.7. Follow up installation to assure that improvements have been achieved.

Flow process charts are used to review and critically examine the overall sequence of anoperation by focusing on the movements of the operator or the flow of materials. The followingfigure describes the symbols used in constructing a flow process chart.


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The following figure illustrates a flow process chart.


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Some representative questions to generate ideas for improvements are:

Why is there a delay or storage at this point?

How can travel distances be shortened or avoided?

Can materials handling be reduced?

Would a rearrangement of the workplace result in greater efficiency?

Can similar activities be grouped?

Would the use of additional or improved equipment be helpful?

Does the worker have any idea for improvements?

A work-machine chart is helpful in visualizing the portions of a work cycle during which anoperator and equipment are busy or idle. The analyst can easily see when the operator andmachine are working independently and when their work overlaps or is interdependent. One useof this type of chart is to determine how many machines or how much equipment the operatorcan manage. Among other things, the chart highlights worker and machine utilization. The

following figure presents an example of a worker-machine chart.


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if the proposed method constitutes a major change from the way the job has been performed inthe past, workers may have to undergo a certain amount of retraining, and full implementationmay take some time to achieve. Follow-up work is needed after a reasonable period and consultagain with the operator.

Motion Study

Motion study is the systemic study of the human motions used to perform an operation. Thepurpose is to eliminate unnecessary motions and to identify the best sequence of motions formaximum efficiency.

Present practice of motions study evolved from the work of Frank Gilbreth, who originated theconcepts in the bricklaying trade in the early 20 th century. Through the use of motions studytechniques, Gilbreth is generally credited with increasing the average number of bricks laid perhour by a factor of 3, even though he was not a bricklayer by trade.

The most-used techniques are:

1. Motion study principles.

2. Analysis of therbligs.3. Micromotion study.4. Charts.

Motion study principles are guidelines for designing motion-efficient work procedures. Theguidelines are divided into three categories: principles for use of the body, principles forarrangement of the workplace, and principles for the design of tools and equipment. Thefollowing table lists some examples of these principles.


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In developing work methods that are motion efficient, the analyst tries to:

Eliminate unnecessary motions.

Combine activities.

Reduce fatigue.

Improve the arrangement of workplace.

Improve the design of tools and equipment.

Therbligs are basic elemental motions. The term therblig is Gilbreth spelled backwards (exceptfor the th). The idea behind the development of therbligs is to break jobs down into minute

elements and base improvements on an analysis of these basic elements by eliminating,combining, or rearranging them. Therblgs are useful for short and repetitive jobs.

A list of common basic elemental motions are:

Search implies hunting for an item with the hands and/or the eyes.Selectmeans to choose from a group of objects.Grasp means to take hold of an object.Holdrefers to retention of an object after it has been grasped.Transport loadmeans movement of an object after hold.Release load means to deposit the object.

Some other therblgs are inspect,position,plan, rest, and delay.

Frank Gilbreth and his wife, Lillian, an industrial psychologist, were also responsible forintroducing motion pictures for studying motions, called micro-motion study. The cost ofmicro-motion study limits its use to repetitive activities, where even minor improvements canyield substantial savings owing to the number of times an operation is repeated.

Motion study analysts often use charts as tools for analyzing and recording motion studies.Activity charts and process charts such as those described earlier can be quite useful. In addition,analysts may use a simo chartto study simultaneous motions of the hands.


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These charts are invaluable in studying operations such as data entry, sewing, surgical and dentalprocedures, and certain assembly operations.

Working Conditions

Working conditions such as temperature, humidity, ventilation, illumination, and noise can havesignificant impact on worker performance in terms of productivity, quality of output, andaccidents.


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Temperature and Humidity. Work performance tends to be adversely affected if temperaturesare outside a very narrow comfort band. That comfort band depends on how strenuous the workis; the more strenuous the work, the lower the comfort range. Heating and cooling are less of aproblem in offices than in factories. Solutions range from selection of suitable clothing to spaceheating or cooling devices.

Illumination. The amount of illumination required depends largely on the type of work beingperformed; the more detailed the work, the higher the level of illumination needed. Otherimportant considerations are the amount of glare and contrast. Sometimes natural daylight can beused as a source of illumination. On the down side, the inability to control natural light (e.g.,cloudy days) can result in dramatic change in light intensity.

Noise and Vibration. Noise can be annoying or distracting, leading to errors and accidents. Itcan also damage or impair hearing if it is loud enough. Successful sound control begins withmeasurement of the offending sounds. Selection and placement of equipment can eliminate orreduce potential problems. In some instances, acoustical walls and ceilings or baffles that deflectsound waves may prove useful. Vibrations can be a factor in job design even without a noisecomponent, so merely eliminating sound may not be sufficient. Corrective measures includepadding, stabilizers, shock absorbers, cushioning, and rubber mountings.

Work Breaks. The frequency, length, and timing of work breaks can have significant impact onboth productivity and quality of output. An important variable in the rate of decline of efficiencyand potential effects of work breaks is the amount of physical and/or mental requirements of thejob.

Safety. Worker safety is one of the most basic issues in job design. Workers can not beeffectively motivated if they feel they are in physical danger.

From an employer standpoint, accidents are undesirable because they are expensive (insuranceand compensation); they usually involve damage to equipment and/or products; they require

hiring, training, and makeup work; and they generally interrupt work. From a worker standpoint,accidents mean physical suffering, mental anguish, potential loss of earnings, and disruption ofthe work routine.

Two basic causes of accidents are worker careless and accident hazards. Protection involves useof proper lighting, clearly marked danger zones, use of protective equipment, safety devices,emergency equipment, and thorough instruction in safety procedures and use of regular andemergency equipment. Housekeeping is another important safety factor. Posters can be effective,particularly, if they communicate in specific terms on how to avoid accidents.


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The enactment of the Occupational Safety and Health Act (OSHA) in 1970 and the creation ofthe Occupational Safety and Health Administration, emphasized the importance of safetyconsiderations in systems design. Inspections are carried out both at random and to investigatecomplaints of unsafe conditions.

3.5.2 Work Measurement

Job design determines the contentof a job, and methods analysis determines how a job is to beperformed. Work measurement is concerned with determining the length of time it should taketo complete the job.

Job times are vital inputs for manpower planning, estimating labor costs, scheduling, budgeting,and designing incentive systems. The times include expected activity time plus allowances forprobable delays. A standard time is the amount of time it should take a qualified worker tocomplete a specified task, working at a sustainable rate, using given methods, tools andequipment, raw material inputs, and workplace arrangement.

Most commonly used methods of work measurement are: stopwatch time study, historical times,predetermined data, and work sampling.

Stopwatch Time Study

Stopwatch time study was formally introduced by Frederick Winslow Taylor in the late 19thcentury. Today it is the most widely used method of work measurement. It is especiallyappropriate for short and repetitive tasks.

Stopwatch time study is used to develop a time standard based on observations of one worker

taken over a number of cycles. The basic steps in a time study are:

1. Define the task to be studied and inform the worker who will be studied.2. Determine the number of cycles to observe.3. Time the job and rate the workers performance.4. Compute the standard time.

In most instances, an analyst will break all but very short jobs down into basic elemental motions(e.g., reach, grasp) and obtain times for each element. The reasons are:

1. Some elements are not performed in every cycle and the breakdown enables the analystto get a better perspective on them.2. A workers proficiency may not be the same for all elements of the job.

3. Building a file of elemental times that can be used to set times for other jobs.The number of cycles that must be timed is a function of three things: (1) the variability ofobserved times, (2) the desired accuracy, and (3) the desired level of confidence for the estimatedjob time. The sample size needed to achieve that goal can be determined using the formula:


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,

where

= number of standard normal deviations needed for desired confidence,

= sample standard deviation,= desired accuracy percentage,

= sample mean, and

= accuracy or maximum acceptable error.

Typical value of used in this computation are:

Desired Confidence (%) value

90 1.65

95 1.96

95.5 2.00

98 2.33

99 2.58

Very often the desired accuracy is expressed as a percentage of the mean of the observed times.In the alternate formula above, the desired accuracy is stated as an amount (e.g., within oneminute of the true mean) instead of a percentage.

Development of a time standard involves computation of three times: the observed time (OT),the normal time (NT), and the standard time (ST).

The observed time is simply the average of the recorded time.

,

where

OT= observed time,

= sum of recorded times, and

= number of observations.

Note: If a job element does not occur each cycle, its average time should be determinedseparately and that amount should be included in the observed time, OT.

The normal time is the observed time adjusted for worker performance. It is computed by

multiplying the observed time by aperformance rating.

or ,

where

NT= normal time,PR= performance rating,

= average time for element , and


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= performance rating for element ,

depending on whether a rating is made for the entire job or on an element-by-element basis.

Obviously, there is room for debate about what constitutes normal performance, andperformance ratings are sometimes the source of considerable conflict between labor and

management. Although no one has been able to suggest a way around these subjectiveevaluations, sufficient training and periodic recalibration by analysts using training films canprovide a high degree of consistency in the ratings of different analysts.

The standard time for a job is the normal time plus an allowance for regular delays.

or ,

where

A= allowance percentage based on job time or workday.

The following table provides typical allowance percentages for working conditions.


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Note: If an abnormal short time has been recorded, it typically would be assumed to be the resultof observational error and thus discarded. However, if an abnormally long time has beenrecorded, the analyst would want to investigate that observation.

Historical (Standard Elemental) Times


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Over the years, a time study department can accumulate a file of elemental times that arecommon to many jobs. After a certain point, many elemental times can be simply retrieved fromthe file, eliminating the need for analysts to go through a complete time study to obtain them.

If there is no exact match between the observed element and the historical element, it is often

possible to interpolate between values on file to obtain the desired time estimate.

Advantages of using historical times are:

Potential savings in cost and effort, Less interruption of work, and Not requiring performance rating adjustment.

A major disadvantage is that times may not exist for enough standard elements to make itworthwhile and the times may be biased or inaccurate.

Predetermined Data (Time Standards)

Predetermined time standards involve the use of published data on standard elemental times.A commonly used system is methods-time measurement(MTM), which was developed in thelate 1940s by the Methods Engineering Council.


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To use this approach, the analyst must divide the job into its basic elements, measure thedistances involved, rate the difficulty of the elements, and then refer to the appropriate table ofdata to obtain the time for the elements. The standard time for the job is obtained by adding thetimes for all of the basic elements.


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Time of the basic elements are measured in time measurement units (TMUs); one TMU equals0.0006 minutes.

A high level of skill is required to generate a predetermined time standard. Analysts generallytake training or certification courses to develop the necessary skills to do this kind of work.

Among the advantages of predetermined time standards are the following:

They are based on a large numbers of workers under controlled conditions.

The analyst is not required to rate performance in developing the standard.

There is no disruption of the operation.

Standards can be established even before a job is done.

But, some argue that many activity times are too specific to a given operation to be generalizedfrom published data. Others argue that different analysts perceive elemental activities andevaluate their difficulty differently.

Work Sampling

In work sampling, an observer makes brief observations of a worker or a machine at randomintervals and simply notes the nature of the activity. The resulting data are counts of the numberof times each category of activity or non-activity was observed.

Two primary uses of work sampling are ratio-delay study and analysis of non-repetitive jobs.

Work sampling is designed to provide a value, , which estimates the true proportion,p, within

some allowable error, e, i.e., . The variability associated with sample estimates ofp tends

to be approximately normal for large sample sizes. Consequently, the normal distribution can beused to construct a confidence interval and to determine required sample size.

The amount of the maximum probable error is a function of both the sample size and the desiredlevel of confidence. For large samples, the maximum error e (in percentage) can be computedusing the formula


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,

where

z= number of standard deviations needed to achieve desired confidence,

= sample mean, and

n= sample size.

By solving the above formula for n, we can determine the appropriate sample size using theconfidence level and the amount of allowable error provided by management.

When no sample estimate ofp is available, a preliminary estimate of sample size can be obtainedusing =0.5. If the resulting value ofn is non-integer, round it up.

The overall procedure of work sampling is:

1. Clearly identify the worker(s) or machine(s) to be studied.2. Notify the workers and supervisors of the purpose of the study to avoid arousing

suspicious.3. Compute an initial estimate of sample size using a preliminary estimate ofp, if available.

Otherwise, use =0.5.4. Develop a random observation schedule.5. Begin taking observations. Recompile the required sample size several times during the

study.6. Determine the estimated proportion of time spent on the specified activity.

The following table provides a comparison of work sampling and time study.


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3.5.3 Compensation

Organizations use two basic systems for compensating employees: time-based systems andoutput-based systems. Time-based systems, also known as hourly and measured day-worksystems, compensate employees for the time the employee has worked during a pay period.Output-based (incentive) systems compensate employees according to the amount of outputthey produce during a pay period, thereby tying pay directly to performance.

The following table gives a comparison of both systems.


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Individual incentive plans typically incorporate a base rate that serves as a floor. The simplestplan is straight piecework. Workers are guaranteed that amount as a minimum, regardless ofoutput. The base rate is tied to an output standard; a worker who produces less than the standardwill be paid at the base rate. This protects workers from pay loss due to delays, breakdowns, andsimilar problems. In most cases, incentives are paid for output above standard, and the pay is

referred to as a bonus.

A variety of group incentive plans, which stress sharing of productivity gains with employees,are in use.

Scanlon Plan. The plan encourages reduction in labor costs by allowing workers to sharethe gains from any reduction achieved.

Kaiser Plan. The plan encourages reduction in labor, material, and supply costs withsavings shared by employees.

Lincoln Plan. The plan includes profit sharing, job enlargement, and participative

management. The components of the plan are a piecework system, an annual bonus, anda stock purchase provision.

Kodak Plan. This plan uses a combination of premium wage levels and an annual bonusrelated to company profits instead of more traditional incentives. Workers are encouragedto help set goals and to decide on reasonable performance levels. The idea is that theirinvolvement will make workers more apt to produce at a premium rate.


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Knowledge-based pay systems award workers based on the knowledge and skill which theworkers possess. The systems have three dimensions: horizontal skills reflect the variety of tasksthe worker is capable of performing; vertical skills reflect management tasks the worker iscapable of; and depth skills reflect quality and productivity results.

Management compensation is closely tied to the success of the company or division that an

executive is responsible for.

3.5.4 Learning Curves and Analysis (T. P. Wright, 1936)

Whenever humans are involved, learning is a basic consideration in the design of work systems.Learning curves show that the time required to perform a task decreases with increasingrepetitions. Note that the time reduction per unit becomes less and less as the number ofrepetitions increase.

If a number of changes is made during production, the learning curve would be more realisticallydescribed by a series of scallops instead of a smooth curve.

From an organizational standpoint, what makes the learning effect more than an interestingcuriosity is its predictability, which becomes readily apparent if the relationship is plotted on alog-log scale.


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The straight line reflects a constant learning percentage, which is the basis of learning curveestimates: every doubling of repetitions results in a constant percentage decrease in the time perrepetition. Typical decreases range from 10 percent to 20 percent.

By convention, learning curves are referred to in terms of the complements of their improvementrates. For example, an 80 percent learning curve denotes a 20 percent decrease in unit (oraverage) time with each doubling of repetitions.

There are two ways to obtain the exact time per unit for a given number of repetitions. One is theformula approach using the following standard formula of learning curves.

,

where

= time for nth unit,

= time for the first unit,

n= number of units, and


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b= the learning percent; ln stands for the natural logarithm.

The other approach is to use a learning factor obtained from the following table. If for somereason the completion time of the first unit is not available, or if the manager believes thecompletion time for some later unit is more reliable, the table can be used to obtain an estimate

of the initial time.

Learning curve theory has found useful applications in a number of areas, including:

Manpower planning and scheduling. Negotiated purchasing. Pricing new products. Budgeting, purchasing, and inventory planning. Capacity planning.

Learning curves often have strategic implications for market entry, when an organization hopesto rapidly gain market share. An increase in market share creates additional volume, enablingoperations to quickly move down the learning curve, thereby decreasing costs and, in the


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process, gaining a competitive advantage. In some instances, the volumes are sufficiently large,the operations will shift from batch mode to repetitive operation, which can lead to further costreductions.

Managers using learning curves should be aware of the following limitations and pitfalls.

1. It is best to base learning rates on empirical studies rather than assumed rates wherepossible.

2. Projections based on learning curves should be regarded as approximations of actualtimes and treated accordingly.

3. It may be desirable to revise the base time, as later times become available.4. It is possible that at some point the curve might level off or even tip upward, especially

near the end of a job. Some of the better workers or other resources may be shifted intonew jobs that are starting up.

5. Improvements in times may be caused in part by increases in indirect labor costs.6. In mass production situations, learning curves may be of initial use in predicting how

long it will take before the process stabilizes. For the most part, however, the conceptdoes not apply to mass production, because the decrease in time per unit is imperceptiblefor all practical purposes.

7. Uses of learning curves sometimes fail to include carryover effects; previous experiencewith similar activities can reduce activity times, although it should be noted that thelearning rate remains the same.

8. Short product life cycles, flexible manufacturing, and cross-functional workers can affectthe ways in which learning curves may be applied.

3.6 Location Planning and AnalysisThe need for location decisions:

Opportunities for expanding marker shares. Business growth. Depletion of basic inputs. Market shift.


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Operating costs.The nature of location decisions:

Entail a long-term commitment.

Select a location from a number ofacceptable locations instead of identifying the onebestlocation. Location options include expansion, addition, closing, or doing nothing.

Location decision making procedure:

1. Decide on the criteria to use for evaluating location alternatives.2. Identify important factors.3. Develop location alternatives:

a. Identify a general region for a location.b. Identify a small number of community alternatives.c.

Identify site alternatives among the community alternatives.4. Evaluate the alternatives and make a decision.

New firms typically locate in a certain area simply because the owner lives there. Largeestablished companies, particularly those that already operate in more than one location, tend totake a more formal approach.

Factors that affect location decisions:


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Multiple plant manufacturing strategies:

1. Product plant strategy. Entire products or product lines are produced in separated plants.Each plant supplies to entire market. The strategy often results in economics of scale.

2. Market area plant strategy. Plants are designed to serve a particular geographic segmentof a market. Individual plant produce most products. This strategy significantly saves onshipping costs.

3. Process plant strategy. Different plants concentrate on different aspects of a process. Thisstrategy is best suited for products that have numerous components. Coordination ofproduction throughout the system becomes a major issue and requires a highly informed

and centralized administration to achieve effective operation.

Service and retail locations typically place traffic volume and convenience high on the list ofimportant factors. Available public transportation and parking facility are often a consideration.

Factors relating to foreign locations:

The future will see a trend toward a smaller and automated factories, microfactories, with a

narrow product focus located close to markets.

3.6.1 Evaluating Location Alternatives

Locational Cost-Profit-Volume Analysis

Graphical assumption:

1. Fixed costs are constant for the range of probable output.2. Variable costs are linear for the range of probable output.3. The required level of output can be closely estimated.4. Only one product is involved.

Graphical Procedure:


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1. Determine the fixed and variable costs associated with each location alternative.2. Plot the total-cost lines for all location alternatives on the same graph.

where

FC= fixed cost

VC= variable cost per unit

Q= quantity or volume of output

3. Determine which location will have the lowest total cost for the expected level of output.Alternatively, determine which location will have the highest profit.

For a profit analysis, compute

,

where

R= revenue per unit.

When a problem involves shipment of goods from multiple sending points to multiple receivingpoints, and a new location (sending or receiving point) is to be added to the system, the companyshould undertake a separate analysis of transportation. In such instances, the transportationmodel of linear programming is very helpful.

Factor Rating


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Factor rating establishes a composite value for each alternative that summarizes all relatedfactors.

The following procedure is used to develop a factor rating:

1.

Determine which factors are relevant.2. Assign a weight to each factor that indicates its relative importance compared with allother factors. Typically, weights sum to 1.00.

3. Decide on a common scale for all factors.4. Score each location alternative.5. Multiply the factor weight by the score for each factor, and sum the results for each

location alternative.6. Choose the alternative that has the highest composite score.

In some cases, managers may prefer to establish minimum thresholds for composite scores. If analternative fails to meet that minimum, they can reject it without further consideration.

The Center of Gravity

The center of gravity method treats distribution cost as a linear function of the distance and thequantity shipped.

The quantity to be shipped to each destination is assumed to be fixed. An acceptable variation isthat quantities are allowed to be different, as long as their relative amounts remain the same.

The coordination of the center of gravity is defined as

,

where

= quantity to be shipped to destination i,


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=x coordinate of destination i, and

=y coordinate of destination i.

If the quantities to be shipped to every location are equal, you have

,

where

n= number of destinations.

a. Map showing destinations.

b. Add coordinates.

c. Center of gravity.


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3.6.2 Transportation Model

The transportation problem involves finding the lowest-cost plan for distributing stocks of goods

or supplies from multiple origins to multiple destinations that demand the goods.

The following figure shows the nature of a transportation problem in real life.

The information needed consists of the following:

1. A list of origins and ones capacity or supply quantity per period.2. A list of the destinations and each ones demand per period.


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3. The unit cost of shipping items from each origin to each destination.The information is arranged into the following transportation table.

Assumptions are:

1. The items shipped are homogeneous.2. Shipping cost per unit is the same regardless of the number of units shipped.3. There is only one route or mode of transportation being used between each origin and

each destination.

Major steps in solving the transportation using the table are:

1. Obtaining an initial solution.2. Testing the solution for optimality.3. Improving sub-optimal solutions.

Obtaining An Initial Solution --- The Intuitive Lowest-Cost Approach

The procedure involves these steps:

1. Identify the cell with the lowest cost.2. Allocate as many units as possible to that cell, and cross out the row or column (or both)

that is exhausted by this.3. Find the cells with the next lowest cost from among the feasible cells.4. Repeat steps (2) and (3) until all units have been allocated.


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Testing For Optimality --- Stepping Stone

In the stepping-stone method, cell evaluation proceeds by borrowing one unit from a full cell andusing it to assess the impact of shifting units into the empty cell. Helpful rules for obtainingevaluation paths are

1. Start by placing a + sign in the (empty) cell you wish to evaluate.2. Move horizontally (or vertically) to

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Part 3 Production System Design