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8/13/2019 Mb0044 Unit 13 Slm
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Production and Operations Management Unit 13
Sikkim Manipal University Page No. 309
Unit 13 Operations Scheduling
Structure:
13.1 Introduction
Objectives
13.2 Purpose of Operations Scheduling
13.3 Factors Considered while Scheduling
13.4 Scheduling Activity under PPC
13.5 Scheduling Strategies
Detailed scheduling
Cumulative scheduling
Cumulative-detailed combination
Priority decision rules
13.6 Scheduling Guidelines
13.7 Approaches to Scheduling
Forward scheduling
Backward scheduling
13.8 Scheduling Methodology [Quantitative]
Charts and boards
Priority Decision Rules
13.9 Scheduling in Services
13.10 Summary
13.11 Glossary
13.12 Terminal Questions
13.13 Answers
13.1 Introduction
In the previous unit, we have dealt with the concepts of supply chain
management, domain applications, views on supply chain, bullwhip effect in
SCM, collaborative supply chain, inventory management in supply chain,
and financial supply chain. In this unit, we will deal with the purpose of
operations scheduling, factors considered while scheduling, scheduling
activity under PPC, scheduling strategies, scheduling guidelines,
approaches to scheduling, scheduling methodology, and scheduling in
services.
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Production is the transformation of inputs to the desired outputs. The
highest efficiency of the production is obtained by manufacturing therequired quantity, to the required quality, at the required time, and with the
best processes. To achieve these, the production management employs
Production Planning and Control department, known as PPC, whose
function is to coordinate all the production activities.
As the name of PPC represents, planning activity is the choice from several
alternatives of utilising the existing resources to achieve the desired
objectives and the control is monitoring of performance through the
feedback by comparing the results accomplished against the target and
taking corrective actions to improve the performance. Operations are the
processes performed on machines to achieve the desired results. Hence,PPC is a tool for directing all manufacturing operations, coordinating and
ensuring the end results of desired quantity, quality, time, and cost of
production. PPC functions are wide spread in planning and control of
materials, methods, machines, manpower, routing, estimating, scheduling,
dispatching, expediting, and evaluating the output.
Scheduling is the assignment of work to the production units with detailed
specifications of the times and the sequence of manufacturing until the end
product is rolled out and ready for dispatch. It also provides the performance
yardsticks on the time required to perform each operation and for the entire
series of activities as routed. Scheduling refers to firming up processing
times so that all the jobs/tasks are completed by the time they are due for
delivery to customers.
Objectives:
After studying this unit, you should be able to:
describe how scheduling aims at achieving the required output with
minimum of delay and disruption in processing
explain how it provides the means to maximise utilisation of men,
machines, and materials by maintaining free flow of materials along the
production lines and units identify how it helps to prevent the unbalanced allocation of time among
work centres with the view to eliminate idle capacity
recognise how it keeps the production cost to minimum
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13.4 Scheduling Activity under PPC
Table No 13.1: Terminologies
Routing Routing is the planning activity undertaken to determine the
best route for manufacturing a product. It lays down the flow
of work in the plant and determines what work is to be done,
where, and how. Routing considers plant layout, storage
space for inventory, and material handling. Route sheets
are developed for each job.
Scheduling Determines the order of processing jobs at each work
centre and establish their start and finish times (Discussed
in detail below)
Dispatch Allow production to commence through supply of materials
and work orders
Follow up Monitor progress and take corrective actions to minimise
deviations
13.5 Scheduling Strategies
Scheduling strategy differs from organisation to organisation as it depends
on the quantum of production, size and type of production, company’s
policy, priorities, etc. Most of these strategies are concerned with job shop
production since the problems encountered is more when more than one
product is produced in the same plant.
Following are the classifications:
Detailed scheduling
Cumulative scheduling
Cumulative-detailed scheduling
Priority decision rules
Let us now discuss these classifications in detail.
13.5.1 Detailed scheduling All job orders from customers are scheduled to the last details. This may not
be practical in case disruptions are there in production line like machine
breakdown, absenteeism, etc. (Possible in airlines, hotels, etc)
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13.5.2 Cumulative scheduling
The customer orders are pooled to form a cumulative work load and thenmatched with the capacity. The work load is then allocated in such a way
that immediate periods get allocated to maximum capacity.
13.5.3 Cumulative-detailed combination
This combines both the earlier strategies of firm and flexible nature of work
load. Cumulative work load projections can be used to plan for capacity as
needed. As changes happen during the week, the materials and capacity
requirements are updated. The actual time allocated to the specified job at
each work centre is as per the standard hours needed. This is tuned further
with the requirements of the master schedule.
13.5.4 Priority decision rules
When a set of orders are to be executed, the question of prioritising arises.
These priority decision rules are scheduling guidelines used independently
or in conjunction with any one of the above three strategies. These are
discussed later in section 13.8.2.
13.6 Scheduling Guidelines
1) Provide a realistic schedule
A schedule should be realistic rather than idealistic considering all the
practical possibilities.2) Allow adequate time for operations
Enough time should be allowed for production.
3) Allow adequate time before, between, and after operations
Enough time should be allowed to queuing and transit of Work in
Progress or FIG.
4) Don’t release all available jobs to the shop
Releasing all the available jobs as and when they are received
overloads the capacity. It also increases the lead time and excess work-
in-process inventory.
5) Don’t schedule all available capacity in the shop
Some excess capacity should be available to handle emergencies and
sudden alteration in jobs or to accommodate a totally new but profitable
order.
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6) Load only selected work centres
Only those work centres which are fit are selected for operations.7) Allow for necessary changes
Schedules should be open to necessary changes and alterations in the
products.
8) Gear up the entire shop to be responsible for the schedule
It is the worker’s responsibility to cooperate and meet the schedule.
13.7 Approaches to Scheduling
There are two types of approaches to scheduling. They are forward
scheduling and backward scheduling. These are used to ensure that the
lead time for manufacture is kept to a minimum and the products are
supplied to the customers as quickly as possible.
13.7.1 Forward scheduling
Forward scheduling is an approach where the customer orders are
immediately processed as soon as they come in even if their due dates are
far away. With forward scheduling, the scheduler selects a planned order
release date and schedules all the activities from this point forward and
ready within time.
13.7.2 Backward scheduling
With backward scheduling, the scheduler begins with a planned receipt dateor due date and moves backward in time according to the required
processing times. It is an approach where the customer orders are
processed as late as possible so that they are finished and delivered exactly
on their due dates. Here the starting time of the processing job is
determined by “setting back” the number of days required for its processing,
from finish date.
Illustration 1: Suppose all processes required for a component takes 20
days, then forward and backward scheduling can be executed as follows:
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Forward scheduling
End of operations Delivery due date
Work forward
Opns 01 02 03 04 05 06 07 08 (Finished Goods Inventory
Days: 2 5 6 9 11 13 15 17 (three days left for delivery)
Backward Scheduling:
Start of Operations (start late by three days) Delivery due date
End of operations
Operations 01 02 03 04 05 06 07 08 FGI
Days: 2 5 6 9 11 13 15 17
Work backward
Fig. 13.1: Forward and backward scheduling
13.8 Scheduling Methodology [Quantitative]
There are many types of scheduling and the methodologies used in
production. Specific methods used depend on the type of industry,
organisation, products, and level of sophistication in the production.
However the scheduling methodology can be classified as (i) charts and
boards and (ii) priority decision rules.
13.8.1 Charts and boards
a) Gantt load chart
b) Gantt progress chart
c) Schedule boards
d) Computer graphics
Let us now discuss these methodologies in brief.
a) Gantt load chart
A Gantt load chart shows the amount of cumulative workload that each work
centre has in a manufacturing unit. It is a graph showing individual and total
estimated workloads of each work centre on a time scale.
Uses of Gantt chart
Total workload shown graphically is simple, clear, and easy to
understand.
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It indicates the need for more resources or for reassigning or resources
when the load at one work centre becomes too large. In case some work centres are overloaded, employees from a low-load
work centre may be temporarily shifted to high load centres.
Limitations of Gantt chart
Gantt load chart does not account for delays and disruptions at the work
centres
It does not give information regarding the due date requirements of each
job
Chart must be updated periodically for new jobs Illustration 2
Work Centre Cumulative Work Loads
Sheet
Metal
A (5) B(16) C(14) D(9) E(11)
0 5 21 35 44 55
SheeringBending
A(4) B(12) D(24)
0 4 16 40
Painting A(5) B(7) C(12) D(4) E(6)
0 5 12 24 28 34
Brazing C(8) E(16)
0 8 16
Fig. 13.2: Work centres & the cumulative work load
In a production facility where five jobs to be processed through one or more
of the four work centres, the cumulative work load can be shown in the chart
given above (Fig 13.2). The work centre “Sheet Metal” has cumulative work
load of 55 days to complete all the five jobs (A to E). Similarly the
cumulative load for completion of three jobs in work centre ‘Shearing and
Bending’ is 40 days. The painting section having 5 jobs will take 34 days
and brazing section for only two jobs will have cumulative work load of 16
days. This chart will indicate only the cumulative workloads for each job and
to what extent the work centres are to be scheduled.
b) Gantt schedule and progress chart
A Gantt schedule and progress chart indicates the scheduled starting and
completion dates for each job as well as the current status of each job. The
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chart shows an immediate comparison between the schedule and reality
with respect to each job at work centres.Illustration 3
There are five machines with four jobs to be finished. The work schedule for
sheet metal work and soldering on job X is 4 and 4 days. The schedule for
braze on Y job is 5 days, Z job is scheduled for sheet metal wok for 6 days
and a finishing job of soldering a job F is of 3 days. (See Fig 13.3)
Job Machine 1 2 3 4 5 6 7 8 9 10 11
X(4) Sheet
work-1
X(4) Solder
Y(5) Braze
Z(6) Sheet
work-2
F(3) Solder
Fig. 13.3: Progress chart
Jobs are shown in Column one and the work completed on a particular days
in the progress chart above, is shown As against 4 days of sheet metal work-1 of X component, the operation has
completed up to 3.5 days, and hence there is half a day work pending on
that day.
a) Job X requires half day of change over on 5th day-shown as Black
square and then, soldering operation is continued for next four days.
b) Job Y Brazing continued for five days and as on this day it is as per
schedule, i.e., it has taken 5 days to complete as scheduled.
c) Since component X requires half day for completion before component
Z starts its operation on the sheet metal machine, six days required for
this Z will take completion date to 10.5 days.
d) Soldering operation on component F has already taken half a day extra
for completion (3.5 days instead of 3 days).
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This is how the progress charts shows the present day status of the job on
hand, jobs completion, jobs within the schedules, and jobs outside theschedule.
c) Schedule boards
Shop floor personnel need to know the information as to how they are
processing components in production and this can be reflected in a simple
schedule board inside the production shops. Content of these boards are
updated daily by the progress department. The boards contain simple bar
graphs to represent the actual status of components/products.
d) Computer graphics
Computer graphics and reports have replaced the mechanical boards and
charts. With the help of computers, PPC keep track of hundreds of itemsand can revise the schedules.
13.8.2 Priority decision rules
A priority decision rules shown below are the systematic procedures for
assigning priorities to waiting jobs, and determining the sequence in which
jobs are required to be processed.
The major criteria for applying rules are set up costs, idle time of machine
and labor, in-process inventory, percentage of jobs that are late, average
number of jobs waiting in queue, average time to complete job, and
standard deviation of time to complete job.Classifications of priority decision rules
A. Single-criteria rules
B. Combined criteria rules (Johnson’s rule)
C. Critical ratio scheduling
D. Index method of scheduling
E. Critical path method
Let us now discuss these classifications in detail.
A) Single-criteria rules
Here the jobs are assigned to the production division by considering singleand important criteria. These criteria’s are:
First come first served (the job that comes first is served by scheduling
first)
Earliest due date (here the job with the earliest due date is processed)
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Least slack available for production (here priority is given to the waiting
job whose slack time is least. Slack time is calculated as the differenceof the length of time remaining until the job is due and the length of its
operation time. For example, if a job requires 6 days and time left is 8
days, then the slack for that job is 2 days.)
Shortest processing time (job requires least of shortest time is
processed first)
Longest processing time (job that requires the longest time is processed
first)
Preferred customer order (priority to orders coming from favorite
customers)
Random selection (Jobs are selected at random and purely a chance forany job)
Illustration - 4
The illustration below details how the above said priority rules are applied
while scheduling. It also explains the different methods available for PPC
department.
Five jobs are to be processed at a Fabricating unit. The processing time in
days and the due date in days are given below.
Table 13.1: Data for illustration 4
Job A B C D E
Processing time in days 8 21 18 13 15
Due date in days 10 24 22 16 16
Determine:
a) The sequence of job according to the shortest processing time
b) Calculate total completion time
c) Calculate mean flow time or average completion time
d) Calculate average number of jobs in the system each day
e) Calculate average lateness
Solution
a) The sequence as is ABCDE, where as the sequence with the shortest
flow time is ADECB
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Table 13.2: Computation of flow time days and late days
JobSequence ProcessingTime in days Total Flowtime days Due daysfrom now Late(days)
A 8 8 10 -2
B 21 29 (8+21) 24 5
C 18 47 (29+18) 22 25
D 13 60 (47+13) 16 44
E 15 75 (60+15) 16 59
Total 75 219
b) Total completion time = Total process days = 75 days
c) Average number of jobs running at a time = Total cumulative flow time insequence at a time/total process days required. Here when the first job
is processed other four jobs are waiting. Similarly when the first job is
delivered, the second job is processed and the other three jobs are
waiting. This type of elimination goes ahead until all the five jobs are
completed.
Therefore, the average number of jobs in the system each day:
Avg No. of Jobs = [ (5x8) + (4x21) + (3x18) + (1x15) ] / (75)
= 2.92 jobs in the system/day
d) Sequencing jobs using short processing time (SPT)
This can be can be evolved by rearranging the above table as
ascending order of the processing time. This ascending order will be
ADECB, as shown below:
Table 13.3: Computation of flow time days and due days
Waiting Job -SPT Processingtime
Flow time indays
Due Date
A 8 8 10
D 21 29 (8+21) 24
E 18 47 (29+18) 22
C 13 60 (47+13) 16
B 15 75 (60+15) 16
Total 75 219
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Average/Mean Flow time = Total cumulative flow time in
days/number of sequences= 219/5
= 43.8 days.
This means the average job in the production line is 43.8 days.
e) Average job lateness = Cumulative late in days/no of sequences
= [0+5+25+44+59] / 5 = 26.6 days.
Here for A the lateness is (-2), i.e., produced 2 days earlier, hence
lateness is zero.
B) Combined criteria rules (Johnson’s rule)
Johnson’s rule is used to determine the sequence of order for a series of
jobs to be processed on a fixed number of machines. The basis for the
sequencing is that the total time required to complete all the jobs should be
minimum thereby reducing the idle time of all machines. Johnson’s rule is a
procedure that minimises the total cycle time in scheduling a group of jobs
on two workstations and the sequence of the jobs at the two work stations
should be identical and hence priority assigned to a job should be the same
at both. This type of production sequence for a group of jobs to minimise the
time has two advantages:
1) The group of jobs is completed in minimum time
2) Utilisation of two station flow shop is maximised
Steps used to adopt Johnson’s rule
Step - 1: List the processing/operation times from ‘n’ jobs on the two
work centres/machines.
Step - 2: Scan all the processing/operation times for the ‘n’ jobs on both
machines and select the shortest processing/operation time in
either work centre.
Step - 3: If the shortest processing/operation time happens to be for a
job on the first work centre/machine, place that job first in the
sequence. If it is at the second work centre/machine, then
place the job last in the sequence.
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Step - 4: Remove the job assigned to the sequence in step 3 from
further consideration (i.e. cross off both times for the assigned job).
Step - 5: Repeat steps 2, 3, and 4 until all the jobs are assigned into the
sequence.
Note: In case of a tie (i.e. a job having the same operation time on both the
machines or two jobs having the same operation time on either of the
machines), choose the job with the smallest subscript first for assigning into
sequence (i.e. when job A and job B has the same time on either machine
M1 and M2, choose job A first as compared to job B). On the other hand if
any job has the same processing/operation time on machines M1 and M2,
then give preference to the processing time/operation time on M1 to be
considered first for assigning that job into the sequence. The above rules
are known as ‘Thumb Rule’ to be followed in case of tie between two jobs or
between two machines.
Illustration - 5
Two machines working for six jobs to be produced whose time for two
operations are given below. Sequence them for scheduling by adopting
Johnson’s rule and find the minimum elapsed time and idle time on each
machine.
Table 13.4: Data for illustration 5Job Machine A
Time in hours
Machine BTime in hrs
1 7 12
2 16 14
3 9 13
4 6 10
5 13 7
6 15 5
Applying the Johnson’s rule for the above machines: 1) Among all the processing time, irrespective of which machine, the
shortest processing time is selected. In the given table, the shortest time
is for job 6 on machine B, and hence place this job as late as possible,
i.e., last.
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Illustration - 6
In the following table, four jobs are shown with its operation time, due dates,number of operations remaining, and shop remaining time. Calculate the CR
sequence schedule.
Table 13.5: Data for illustration 6
Job Operatingtime (hrs)
Timeremainingdue days
Number ofoperationsremaining
Shop timeremaining
AnswerCR
1 5 14 8 9 1.55
2 10 11 2 10 1.1
3 8 20 10 17 1.176
4 14 8 6 11 0.72
Answer:
Using CR to schedule, CR = [time remaining to due date]/shop time
remaining CR for first reading =14/9 = 1.55. Similar calculation and readings
are filled in the CR column. Arranging sequence is the lowest CR first and
then ascending order and hence sequence of job loading is 4, 2, 3, 1.
D) Index method of scheduling
This assigns job to the best machine until its capacity is exhausted and then
remaining jobs are assigned to the next best machine, etc. Here, the jobs
are assigned to the best work centre till it is fully loaded to capacity and the
remaining jobs are assigned to the next best if processing time is thecriterion. If the jobs can be processed in different work centres, “indices” are
calculated for the different likely process time with the lowest index time of
1.0.
Illustration - 7
Solve the following shop loading problem by using index method.
Table 13.6: Data for illustration 7
Job WC-1 WC-2 WC-3 WC-4
A 10 9 8 12
B 3 4 5 2
C 25 20 14 16
D 7 9 10 9
E 18 14 16 25
No. of days available 20 20 20 20
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The following table 13.6 shows the indices. The days corresponding to a job
assigned is with bold letter and underlined. Bold number and underlined isthe minimum number of days in each centre.
Table 13.7: Indices for illustration 7
WC1 WC2 WC3 WC4
Days Index Days Index Days Index Days Index
A 10 1.25 9 1.13 8 1.00 12 1.5
B 3 1.5 4 2.00 5 2.5 2 1.00
C 25 1.78 20 1.42 14 1.00 16 1.14
D 7 1.00 9 1.28 10 1.42 9 1.28
E 18 1.28 14 1.00 16 1.14 25 1.78
No ofDays
20 20 20 20
Daysassign
7 14 22 2
Indices are decided as: Job A has a minimum processing time of 8 days at
centre 3, and hence, this index is 1.00, i. e., the processing times of 10, 9,
and 12 at centres at 1, 2, and 4 are divided by 8 to find out the indices. This
procedure is followed for all jobs and their indices are indicated in the
column.
Assignment of the jobs to work centres based on the index equal to 1 is asfollows:
Table 13.8: Assignment of the jobs to work centres for illustration 7
Jobs A B C D E
Work centre assigned 1 4 3 1 2
[Additional Job of C can be assigned to work centre 4 as sufficient time
available]
E) Critical path method
It is used for scheduling large and unique projects in which the relationship
between the activities is quite intricate. The method overcomes thedeficiencies of Gantt chart.
Here a network of work centres and processing routes of each job is drawn
graphically. PERT/CPM charts are made to identify the critical path.
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Appointments
Here the customer is specified with the time of services. The advantage isthat the service is customised and utilisation of the capacity will be high. The
individual customer needs are satisfied. Service activities are determined
and planned for the customer. For example, surgery in hospitals.
Self Assessment Questions
1. Which of the following is not a characteristic of forward scheduling?
a) Production starts after the job order is received
b) Start and finish time is found by the earliest time slot available at a
work centre
c) Jobs starts in advance as per the latest available time slot in workcentre
2. In which of the following, the Gantt chart application is not there?
a) Used to track performance of shop floor employees
b) Used to reflect work load levels for machines and workstations
c) Can adopt change in scheduling each work centre
3. While job sequencing, _________ the critical ratio, __________ the
priority is given.
a) higher, higher
b) higher, lower
c) lower, higher4. Which of the following is not a priority rule?
a) Shortest processing time rule
b) Allow longest slack time
b) Earliest due date processing rule
c) Critical ratio rule
5. Which one of these is not a scheduling activity?
a) Dispatch
b) Routing
c) Scheduling
d) Facility planning6. Fill up the blanks with appropriate word/words
a) A priority decision rule is a systematic procedure for _________
_________ to waiting jobs, and determining the sequence in which
jobs are required to be processed.
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b) __________ rule is used to determine the sequence of order for a
series of jobs to be processed on a fixed number of machines.c) Critical ratio = [Time remaining for due date] / [ ________________ ]
13.10 Summary
Let us summarise the key learnings of this unit:
PPC focus on operation control through the process of scheduling.
Scheduling helps in operational controls in manufacturing and service
systems. The factors that greatly influence scheduling are the number of
jobs, the number of machines as well as the complexity of the machining
operations in the line.
There are two types of approaches to scheduling. They are forward
scheduling and backward scheduling. These are used to ensure that the
lead time for manufacture is kept to a minimum and the products are
supplied to the customers as quickly as possible.
The scheduling methodology adopted as well as its complexity differ
from production shop to shop and also in the systems like job shops,
process shops, mass production lines, fully automated plants, etc. There
are many types of scheduling and the methodologies used in production.
Specific methods used depend on the type of industry, organisation,
products, and level of sophistication required in the production.
The scheduling methodology can be classified as charts and boards
(Gantt load and progress chart, scheduled boards, and computer
graphics) and priority decision rules like single and double criteria rules,
critical ratio scheduling, index method, and critical path method. Each
one of these has its own specific applications and PPC will decide based
on the type of scheduling required to accomplish the job with the
minimum time possible, thus saving costly machine hours.
Workforce scheduling reflects the staffing plan into a specific work
schedule for each employee. Scheduling should ensure proper loading
of jobs based on the skills available within the workforce.
13.11 Glossary
Critical path: The critical path is the path which has the largest amount of
time associated with the activities and this represents the minimum time
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required to be considered while scheduling. This critical path time, which is
the minimum required time, has to be included while calculating the leadtime for delivery of products.
Queuing system: Just like queuing for a railway ticket in front of the
counter, the customer waiting in line for the supplies to be made is
considered while preparing an optimal scheduling model in order to ensure
minimum queuing of the customers.
13.12 Terminal Questions
1. What are the factors to be considered by PPC while scheduling?
2. Explain briefly the four classification of scheduling strategies?
3. Distinguish between forward scheduling and backward scheduling?
4. Explain in brief the uses and limitations of Gantt charts used in
Scheduling?
5. Distinguish between Gantt load charts and Gantt scheduling chart?
6. What is priority sequencing and what are the various criteria used in it?
13.13 Answers
Self Assessment Questions
1. (c)
2. (a)
3. (c)
4. (b)
5. (d)
6. a. Assigning priorities
b. Johnson’s
c. Time needed to completed the job
Terminal Questions
1. Refer 13.3
2. Refer 13.53. Refer 13.7
4. Refer 13.8.1
5. Refer 13.8.1
6. Refer 13.8.2
8/13/2019 Mb0044 Unit 13 Slm
http://slidepdf.com/reader/full/mb0044-unit-13-slm 22/22
Production and Operations Management Unit 13
Sikkim Manipal University Page No. 330
Reference:
Frazier, G., & Gaither, N. (2002). Operations Management. South-Western/Thomson Learning.
Ronald, E. J., & Everett, A. E. (2009). Production And Operations
Management: Concepts, Models And Behavior. Phi Learning
E-Reference:
www.enotes.com
www.som.umass.edu
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