Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall. 11 - 1 Chapter 11 Statistically-Based Quality Improvement for Variables

Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall.11 - 1

Chapter 11

Statistically-Based Quality Improvement

for Variables

Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall.11- 2

Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesChapter 11

Statistical FundamentalsProcess Control ChartsSome Control Chart Concepts for VariablesProcess Capability for VariablesOther Statistical Techniques in Quality Management


Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals

Statistical ThinkingAll work occurs in a system of interconnected processesAll process have variation (The amount … tends to be underestimated)Understanding variation and reducing variation are important keys to success



Why do statistics sometimes fail in the workplace?Lack of knowledge about the toolsGeneral disdain for all things mathematicalCultural barriers in a companyStatistical specialists have trouble communicating



Why do statistics sometimes fail in the workplace?Statistics generally are poorly taught, emphasizing mathematical development rather than applicationPeople have a poor understanding of the scientific method



Why do statistics sometimes fail in the workplace?Organizations lack patience in collecting data. All decisions have to be made “yesterday”Statistics are viewed as something to buttress an already-held opinionPeople fear using statistics



Why do statistics sometimes fail in the workplace?Most people don’t understand random variationStatistical tools often are reactive and focus on effects rather than causes



Type I and Type II Errors Type I error

Producers riskProbability that a good product will be rejected

Type II errorConsumers riskProbability that a nonconforming product will be available for sale



Understanding Process VariationRandom variation

Centered around the meanConsistent amount of dispersion



Understanding Process VariationNonrandom variation

“Special Causes”Results from some eventDispersion and average of the process are changing Process that is not repeatable



Understanding Process VariationProcess stability

Random VariationNot nonrandom variationProcess Charts



Understanding Process VariationSampling Methods

Samples are cheaperTake less timeLess intrusiveDestructive tests may destroy the sample



Random Samples Each piece has an equal chance of being selected for inspection

Systematic SamplesAccording to time or sequence

Rational subgroupsA group of data that is logically homogeneousComputing variation between subgroups



Planning for InspectionWhat type of planning will be usedWho will perform the inspectionWho will use in-process inspectionSample sizeWhat critical attributes to be inspected areWhere inspection should be performed



Control PlansRequired part of an ISO 9000 quality management system (QMS)Provide a documented, proactive approach to defining how to respond when process control charts show that a process is out of control



Control Plans


Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Control Charts

Process Control Chart



Variables and attributes control charts

1. Variable

2. Attribute




1. You must understand this generic process for implementing process charts

2. You must know how to interpret process charts


Strategic Quality PlanningStatistically-Based Quality Improvement for Variables

Process Control Charts


3. You need to know when different process charts are used

4. You need to know how to computer limits for the different type of process chart


Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts

A generalized procedure for developing process charts

1. Identify critical operations in the process where inspection may be needed

2. Identify critical product characteristics

3. Determine whether the critical product characteristic is a variable or an attribute


Strategic Quality PlanningStatistically-Based Quality Improvement for Variables

Process Control Charts

A generalized procedure for developing process charts

4. Select the appropriate process control chart

5. Establish the control limits and use the chart to continually monitor and improve

6. Update the limits when changes have been made to the process



Understanding control charts

A control chart is an application of hypothesis testing where:

The null hypothesis is that the process is stable



Process Chart

Hypothesis Testing



x and R chart



Completed x and R chart



x and R chart calculation worksheet


Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts x and R chart calculation worksheet for slide 11-26



Examples where nonrandom situations occur



Calculation worksheet and x

chart



x and R chart using excel



Geometric and hypergeometric distributions



X and MR charts in excel



Example 11-3 using excel



Example 11-4 using excel



Cusum Chart


Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesSome Control Chart Concepts for Variables

Choosing the correct variables control chart



When a process is out of control some corrective action is needed:

1. Identify the quality problem

2. Form the correct team to evaluate and solve the problem

3. Use structured brainstorming

4. Brainstorm to identify potential solutions



When a process is out of control some corrective action is needed:

5. Eliminate the cause

6. Restart the process

7. Document the problem, root cause and solutions

8. Communicate the results



The effects of tampering with the process


Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Capability for Variables

A highly capable process produces high volumes with few or no defects

World-class levels of process capability are measured by parts per million (ppm) defect levels



Six Sigma

A design program which emphasized engineering parts so that they are highly capable



Population and Sampling distributions for Class heights



Capability Studies Two purposes to determine whether

a process is capable

1. To determine whether a process consistently results in products that meet specifications

2. To determine whether a process is in need of monitoring



Example 11-5 Proportion of Product Nonconforming



The difference between capability and stability

A process is capable if individual products consistently meet specification

A process is stable only if common variation is present in the process


Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesOther Statistical Techniques in Quality Management

Interlinking


Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesSummary

You need: To know the generic process for

developing charts To be able to interpret charts To be able to choose which chart to

use The formulas to derive the charts To understand the purposes and

assumptions underlying the charts


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Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall. 11 - 1 Chapter 11 Statistically-Based Quality Improvement for Variables