Statistical Process Control (SPC) Resource Center

Measuring Process Capability

The primary measures of process capability used are the process capability ratio, the process capability index, and the Cpk.

Some organizations also use Process Performance Indicators (Pp and Ppk), which are related to Cp and Cpk.

Process Capability Ratio (Cr)

  • Describes what portion of the specification the process is taking up. The Cr should be no greater than 75% in order for the process to be considered capable.

Process Capability Index (Cp)

  • The inverse of the capability ratio. The Cp should be at least 1.33 in order for the process to be considered capable.

Cpk

  • The capability measure preferred in most industries because it indicates whether it is capable and how well-centered the process is. The Cpk is the smaller of the Cpu (capability of the upper half of the process) and the Cpl (capability of the lower half of the process). Cpk = smaller of {Cpu, Cpl}