Cp

Cp

Capability index comparing the process variation with the width of the specification limits. Its formula is:

where USL is the upper specification limit, LSL is the lower specification limit and s is the standard deviation. Cp is independent of the units of measure.
Since the process variation is approximately 6 standard deviations wide, a Cp of 1 means the process and specification limits are of the same width. This means that the process will only produce good product if it is perfectly centered. Since no process can be maintained perfectly centered all the time, a Cp of 1 results in higher levels of defects. Ideally a Cp of 2 should be obtained.
Cp can only be calculated in the event of two specification limits. Cp does not consider whether the process is centered. Centering is evaluated using Cc and Cpk.