Process Capability

Before start learning about the concepts behind process capability, it is high recommended that you review the section “Process Control VS Process Capability”.

Process Capability Index, Cp:

  • Process Capability is commonly measured in units of process standard deviations. 
  • Cp is the relationship between the process standard deviation and the range between the upper and lower specification limits.

      Cp = (USL - LSL) / 6 σx

           The minimum acceptable value for Cp is considered to be 1.

Process Capability Index, Cpk:

  • Many times people use the capability index, Cpk which relates the process mean to the nominal value of the specification.
           ZUSL=  (USL - μx) / σ​x  ,  ZLSL = (LSL - μx) / σ​x

           We select the minimum of these two values:

           Zmin = min [ ZUSL , - ZLSL]

           We find Cpk index by dividing this minimum value by 3.

           Cpk = Zmin / 3

Commonly, for the process capability to be considered acceptable, Cpk should be  1.00.