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Chapter 11
Statistically-Based Quality Improvement
for Variables
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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
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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
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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
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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
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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
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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
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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
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Understanding Process VariationRandom variation
Centered around the meanConsistent amount of dispersion
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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
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Understanding Process VariationProcess stability
Random VariationNot nonrandom variationProcess Charts
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Understanding Process VariationSampling Methods
Samples are cheaperTake less timeLess intrusiveDestructive tests may destroy the sample
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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
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
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
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesStatistical Fundamentals
Control Plans
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Control Charts
Process Control Chart
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Control Charts
Variables and attributes control charts
1. Variable
2. Attribute
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Control 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
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables
Process 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
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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
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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
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Understanding control charts
A control chart is an application of hypothesis testing where:
The null hypothesis is that the process is stable
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Process Chart
Hypothesis Testing
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
x and R chart
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Completed x and R chart
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
x and R chart calculation worksheet
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts x and R chart calculation worksheet for slide 11-26
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Examples where nonrandom situations occur
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Calculation worksheet and x
chart
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
x and R chart using excel
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Geometric and hypergeometric distributions
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
X and MR charts in excel
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Example 11-3 using excel
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Example 11-4 using excel
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Strategic Quality PlanningStatistically-Based Quality Improvement for Variables Process Control Charts
Cusum Chart
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesSome Control Chart Concepts for Variables
Choosing the correct variables control chart
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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
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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 cause
6. Restart the process
7. Document the problem, root cause and solutions
8. Communicate the results
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesSome Control Chart Concepts for Variables
The effects of tampering with the process
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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
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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
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Capability for Variables
Population and Sampling distributions for Class heights
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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 process is in need of monitoring
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesProcess Capability for Variables
Example 11-5 Proportion of Product Nonconforming
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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
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Strategic Quality PlanningStatistically-Based Quality Improvement for VariablesOther Statistical Techniques in Quality Management
Interlinking
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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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