The Cpk-Index (Critical Process Capability; k: “Katayori”, Japanese for bias) compares the distance from the process center to the nearest Specification Limit (how the process should be) to the process spread (how it is). The short-term standard deviation sST is calculated using short-term data, usually25-50 samples within a short time frame representing short term variation (i.e. days).
Cpk = Min ( (USL – X-bar)/3sST, (X-bar – LSL)/3sST )
- Data are representative of the process
- The process is stable (no special causes)
- Data are normally distributed
- In contrast to the Ppk Index, the Cpk index is used to describe the capability of the process without considering long term variation.