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.

Calculation:

Cp = (USL – LSL) / 6sST

Assumptions:

  • Data are representative of the process
  • The process is stable (no special causes)
  • Data are normally distributed

Attention:

  • 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

Cp