# Cp

Cp is a simple process capability index that relates the allowable spread of the specification limits to the measure of the actual or natural variation of the process. It is expressed mathematically as:

Where is the average of the subgroup ranges and d2 is a tabled value based on the subgroup sample size.

Cp provides an indication of how capable the process is of meeting the specifications expected by the customer.

It is a simple capability index that relates the allowable spread of the specification limits (USL and LSL) to the measure of the actual, or natural, variation of the process.

The process can be analyzed based on the value calculated for Cp as given below.

1.33 <= Cp - Satisfiable enough

In this case, the process variation is less than the specification and the readings will be lying more close to X double bar. The normality curve plotted for this will look sharp and narrow in width.

1.00 <= Cp < 1.33 - Adequate

In this case, the process is just meeting the specification minimum of 0.3% defectives will be made, more if the process is not centered.

Cp < 1.00 -Inadequate.

In this case, the process variation exceeds the specification and more defectives are made. The normality curve plotted for this will look fat in width.