QUESTION
When doing SPC, it is often asked what Cpk actually means. Why are we so concerned with this magical number?
ANSWER
The main thing Cpk tells you is how capable your process is given a feature on your part and set of data with known specifications (tolerances). More specifically, it can be converted to tell you how many defects you will have with that feature if you made 1,000,000 parts.
Below is a table of Cpks and their corresponding sigma and parts per million (PPM) defective amounts. This table assumes a 1.5 sigma shift. Notice that if 6 standard deviations (sigma) of your parts are inside your spec limits, then you will only have 3.4 defects per million parts for this particular feature.
Therefore, a Cpk of 2 = Six Sigma.
Cpk | Sigma
| Percent Good
| PPM Defective
|
.33 | +/- 1
| 30.23% | 697700 |
.66 | +/- 2
| 69.13% | 308700 |
1.0 | +/- 3
| 93.32% | 66810 |
1.33 | +/- 4 | 99.379% | 6210 |
1.66 | +/- 5 | 99.9767% | 233 |
2.0 | +/- 6
| 99.99966%
| 3.4 |