Cp is one of several indices used to measures Process Capability. Cp is used to predict potential process performance:
Where:
USL, LSL 
Upper and Lower Specification Limits 
σ _{R} 
The process standard deviation. It must be estimated by the range method. See Process Performance 
When Cp was originally conceived a spread of 6 standard deviations was 'state of the art' quality. The '6' in the denominator was used so that a Cp value of 1 or greater indicated a 'capable' process. Since then standards have increased and now a minimum Cp value of 1.67 is usually required.
Cpk is a closely associated process capability index.
