The Cp-Index (Process Capability) compares the tolerances of the process (how it should be) to the process spread (how it is). The short-term standard deviation sST is calculated using short term data, usually 25-50 samples within a short time frame – i.e. days.
Cp = (USL – LSL) / 6sST
- Data are representative of the process
- The process is stable (no special causes)
- Data are normally distributed
- In contrast to the Cpk index, the Cp index does not consider the center of the process!
- In contrast to the Pp Index, the Cp index is used to describe the capability of the process without considering long term variation.
Read also: Ppk, Process Capability