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.
