Interpreting a Process Capability Chart

Since process capability is not valid unless the process is stable, always look at a control chart of the data first. Once statistical process control is evidenced, then the histogram and process capability may be analyzed. These results are easily analyzed and displayed using SPC software.

Interpreting the Capability Indices

1. Capability Indices are only valid for processes in statistical control. See also Process Performance

2. Compare the non-normal and normal indices. Capability Indices are quite sensitive to assumptions of the distribution.

3. A Capability index is a statistic, subject to statistical error. (this error may be viewed for a given set of data using the Capability Interval text box in the Analysis Options dialog box). In a study by Pignatiello & Ramberg (Process Capability Indices: Just Say "NO", ASQC 47th AQC), a Monte Carlo simulation involving 1000 different trials of 30 piece samples showed the following:

For true Cp=1.33:

55 trials calculated Cp < 1.10 (5.5%)

196 trials calculated Cp < 1.20 (19.6%)

For true Cp=1.00:

112 trials calculated Cp > 1.20 (11.2%)

43 trials calculated Cp > 1.30 (4.3%)

4. Most practitioners consider a Capable process to be one that has a Cpk of 1.33 or better, and a process operating between 1.0 and 1.33 is "marginal." Many companies now suggest that even higher levels of Cpk be maintained by their suppliers. A Cpk exactly equal to 1.0 would imply that the process variation exactly meets the specification requirements. Unfortunately, if the process shifted slightly, and the out of control condition was not immediately detected, then the process would produce output that did not meet the requirements. Thus, the "extra" .33 allowed for some small process shifts to occur that could go undetected. The Table below provides an indication of the level of improvement effort required in a process to meet these escalating demands, where "PPM Out of Spec" refers to the average defect level measured in parts per million.

Cpk	One-Sided Spec PPM Out of Spec	Two-Sided Spec PPM Out of Spec
0.25	226627	453255
0.5	66807	133614
0.7	17864	35729
1.0	1350	2700
1.1	483	967
1.2	159	318
1.3	48	96
1.4	13	27
1.5	3	7
1.6	1	2
2	0.00099	0.00198