1. Statistically-Based Quality Improvement
Chapter 11
Statistically-Based Quality Improvement
for Variables

2. Statistical Fundamentals Control Charts
Strategic Quality Planning Statistically-Based Quality Improvement for Variables Chapter 11
Statistical Fundamentals
Control Charts
Some Control Chart Concepts for Variables
Process Capability for Variables
A Closer Look at Quality
Other Statistical Techniques in Quality Management

3. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Statistical Thinking
All work occurs in a system of interconnected processes
All process have variation (The amount … tends to be underestimated)
Understanding variation and reducing variation are important keys to success

4. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Why do statistics sometimes fail in the workplace?
Lack of knowledge about the tools
General disdain for all things mathematical
Cultural barriers in a company
Statistical specialists have trouble communicating

5. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Why do statistics sometimes fail in the workplace?
Statistics generally are poorly taught, emphasizing mathematical development rather than application
People have a poor understanding of the scientific method

6. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical 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 opinion
People fear using statistics

7. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Why do statistics sometimes fail in the workplace?
Most people don’t understand random variation
Statistical tools often are reactive and focus on effects rather than causes

8. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Type I and Type II Errors
Type I error
Producers risk
Probability that a good product will be rejected
Type II error
Consumers risk
Probability that a nonconforming product will be available for sale

9. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Understanding Process Variation
Random variation
Centered around the mean
Consistent amount of dispersion

10. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Understanding Process Variation
Nonrandom variation
“Special Causes”
Results from some event
Dispersion and average of the process are changing
Process that is not repeatable

11. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Understanding Process Variation
Process stability
Random Variation
Not nonrandom variation
Process Charts

12. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Understanding Process Variation
Sampling Methods
Samples are cheaper
Take less time
Less intrusive
Destructive tests may destroy the sample

13. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Random Samples
Each piece has an equal chance of being selected for inspection
Systematic Samples
According to time or sequence
Rational subgroups
A group of data that is logically homogeneous
Computing variation between subgroups

14. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Statistical Fundamentals
Planning for Inspection
What type of planning will be used
Who will perform the inspection
What critical attributes to be inspected are

15. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Control Charts
Variables and attributes control charts
You must understand this generic process for implementing process charts
You must know how to interpret process charts

16. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Control Charts
Variables and attributes control charts
You need to know when different process charts are used
You need to know how to computer limits for the different type of process chart

17. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Control Charts
A generalized procedure for developing process charts
Identify critical operations in the process
Identify critical product characteristics
Determine whether the critical product characteristic is a variable or an attribute

18. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Control Charts
A generalized procedure for developing process charts
Select the appropriate process control chart
Establish the control limits and use the chart to continually monitor and improve
Update the limits when changes have been made to the process

19. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Control Charts
Understanding control charts
A control chart is an application of hypothesis testing where:
The null hypothesis is that the process is stable

20. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Some 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?

21. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Some Control Chart Concepts for Variables
When a process is out of control some corrective action is needed:
Identify the quality problem
Form the correct team to evaluate and solve the problem
Use structured brainstorming
Brainstorm to identify potential solutions

22. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Some Control Chart Concepts for Variables
When a process is out of control some corrective action is needed:
Eliminate the cause
Restart the process
Document the problem, root cause and solutions
Communicate the results

23. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Process 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

24. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Process Capability for Variables
Six Sigma
A design program which emphasized engineering parts so that they are highly capable

25. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Process Capability for Variables
Capability Studies
Two purposes to determine whether a process is capable
To determine whether a process consistently results in products that meet specifications
To determine whether a is in need of monitoring

26. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Process 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

27. Strategic Quality Planning Statistically-Based Quality Improvement for Variables Summary
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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Printed in the United States of America.