Foster Ch12

Managing QualityIntegrating the Supply Chain

S. Thomas Foster

Chapter 12Statistically-Based Quality Improvement

for Variables

Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesChapter 12

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 characteristics

3. 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 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 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 capable

1. 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

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