12- 1 © 2007 Pearson Education Managing Quality Integrating the Supply Chain S. Thomas Foster Chapter 12 Statistically-Based Quality Improvement for Variables
12- 1© 2007 Pearson Education
Managing QualityIntegrating the Supply Chain
S. Thomas Foster
Chapter 12Statistically-Based Quality Improvement
for Variables
12- 2© 2007 Pearson Education
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
12- 3© 2007 Pearson Education
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
12- 4© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
12- 5© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
12- 6© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
12- 7© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
12- 8© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
12- 9© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Understanding Process VariationRandom variation
Centered around the meanConsistent amount of dispersion
12- 10© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Understanding Process VariationNonrandom variation
“Special Causes”Results from some eventDispersion and average of the process are changing Process that is not repeatable
12- 11© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Understanding Process VariationProcess stability
Random VariationNot nonrandom variationProcess Charts
12- 12© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Understanding Process VariationSampling Methods
Samples are cheaperTake less timeLess intrusiveDestructive tests may destroy the sample
12- 13© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
12- 14© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Planning for InspectionWhat type of planning will be usedWho will perform the inspectionWhat critical attributes to be inspected are
12- 15© 2007 Pearson Education
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
12- 16© 2007 Pearson Education
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
12- 17© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesControl Charts
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
12- 18© 2007 Pearson Education
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
12- 19© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesControl Charts
Understanding control chartsA control chart is an
application of hypothesis testing where:
The null hypothesis is that the process is stable
12- 20© 2007 Pearson Education
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?
12- 21© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesSome Control Chart Concepts for Variables
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
12- 22© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesSome Control Chart Concepts for Variables
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
12- 23© 2007 Pearson Education
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
12- 24© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Capability for Variables
Six Sigma
A design program which emphasized engineering parts so that they are highly capable
12- 25© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Capability for Variables
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
12- 26© 2007 Pearson Education
Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Capability for Variables
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
12- 27© 2007 Pearson Education
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