2. Chapter 6: The Control Chart For Fraction Rejected
Eng. Mgt. 385
Statistical Process Control
Stephen A. Raper
Chapter 6: The Control Chart For Fraction Rejected

3. Control Charts For Attributes
p chart, the chart for fraction rejected as nonconforming to specifications
np chart, the control chart for number of nonconforming items
c chart, the control chart for number of nonconformities
u chart, the control chart for number of nonconformities per unit

4. Control Chart for Attributes
Control charts to “gauge” level of quality coming from a process
Can be cost advantages versus variable control charts
Provides quality history
Can serve as a “wake-up” call to improve quality

5. Control Charts for Attributes
Fraction rejected p, may be defined as the ratio of the number of nonconforming articles found in any inspection or series of inspections to the total number of articles actually inspected.
The binomial as a probability law applies
Developed control charts development is similar to variable control charts, and interpretation is similar

6. Control Chart for Attributes
Control limits can be developed where n is constant, or variable
The central line, p or pbar, must be known, assumed, or estimated, but is generally held constant
Development of trial control limits is similar development of trial control limits for variables
Time order is necessary
“removal” of out of control points is necessary to finalize control charts

7. Control Charts for Attributes
Determination of the purpose of the p chart
As pertains to 100% inspection
Discover average proportion of nonconforming articles
Bring to the attention of management any changes in the average quality level
Discover out of control high spots and correct
Discover out of control low spots and see if they are due to inspection error, or are legitimate improvements that can be converted into causes of consistent quality improvement
Suggest places for the use of variable charts to diagnose quality problems.

8. Control Charts for Attributes
Type I and Type II error probability issues are the same as control charts for variables, using Table G
Problem solved tend to be easier to understand and interpret.

9. Program Completed University of Missouri-Rolla
Copyright 2001 Curators of University of Missouri