1. The Certified Quality Process Analyst Handbook: Process Capability Measures Ch. 20 1st edition Ch. 14 2nd edition
Presented by Dr. Joan Burtner
Certified Quality Engineer
Associate Professor of
Industrial Engineering and Industrial Management

2. Process Control vs. Process Capability
Is the process subject only to common cause variation?
Control charts include upper control limit, centerline, lower control limit
Monitor process using tests to identify existence of special causes
Process Capability
Only valid for processes that are under statistical control
Measures of Capability Cp
Cpk
Based on specification limits set by designers and observed process mean
IDM 355 Fall 2016
Dr. Joan Burtner, Associate Professor of Industrial Engineering

3. Analysis of Process Capability: Cp Cpk
Analysis conducted on processes that have been shown to be “in-control”
Only common cause variation exists in the range
Only common cause variation exists in the mean
Two standard measures Cp
Compares variability of process to specifications
Cpk
Is the process sufficiently centered?
IDM 355 Fall 2016
Dr. Joan Burtner, Associate Professor of Industrial Engineering

4. Analysis of Process Capability: Pp Ppk CR
Capability Ratio (CR)
Reciprocal of Cp
Lower values are better
Two additional measures that are used when the process is not in statistical control Pp
Compares variability of process to specifications
Ppk
Is the process sufficiently centered?
IDM 355 Fall 2016
Dr. Joan Burtner, Associate Professor of Industrial Engineering

5. Process Capability Calculations
Use Rbar/d2 to estimate σ
Evaluation
Cpk > Definitely Capable
1.00 < Cpk < Possibly Capable
Cpk  Not Capable
IDM 355 Fall 2016
Dr. Joan Burtner, Associate Professor of Industrial Engineering

6. Process Performance Index
Calculations
Evaluation
A Ppk value of 1 means the distance between the average and the nearest specification limit is 3 standard deviations
A Ppk value of 2 means the distance between the average and the nearest specification limit is 6 standard deviations
Higher Ppk values are better
IDM 355 Fall 2016
Dr. Joan Burtner, Associate Professor of Industrial Engineering