managing quality integrating the supply chain - 4th edition

Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. 11 - 1

Chapter 11Statistically-Based

Quality Improvement for Variables

Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. 11- 2

Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesChapter 11

Statistical FundamentalsControl ChartsSome Control Chart Concepts for VariablesProcess Capability for VariablesA Closer Look at QualityOther 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 inspectionWhat critical attributes to be inspected are


Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesControl Charts

Variables and attributes control charts

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 Control Charts

Variables and attributes 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



A generalized procedure for developing process charts

1. Identify critical operations in the process

2. Identify critical product characteristics3. Determine whether the critical

product characteristic is a variable or an attribute


Strategic Quality PlanningStatistically-Based Quality Improvement for Variables 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 chartsA control chart is an application of

hypothesis testing where:The null hypothesis is that the process is

stable


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

Choosing the correct variables control chart

Are the data variable? Is it homogeneous in nature or not

conducive to subgroup sampling?



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

1. Identify the quality problem2. Form the correct team to evaluate

and solve the problem3. Use structured brainstorming4. Brainstorm to identify potential

solutions



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

5. Eliminate the cause6. Restart the process7. Document the problem, root cause

and solutions8. Communicate the results


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



Capability Studies Two purposes to determine whether

a process is capable1. To determine whether a process

consistently results in products that meet specifications

2. To determine whether a is in need of monitoring



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 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

11- 28Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying,

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managing quality integrating the supply chain - 4th edition

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