Question

One of the outcomes from a typical process capability study is a “process capability index”. Like the Dow Jones Index, the higher the number, the better is someone’s satisfaction with that number. How would you explain to a manager what the process capability index indicates and what a 10% improvement in the index would represent in real terms?

Answer 1

Process Capability Index, C_pk, is a statistical measure of the ability of a process to produce output within specification limits. It measures producer's capability to produce a product within customer's tolerance range.

Formula for Process Capability Index:

,

where

= mean of the normal data

= SD of normal data

USL = Upper Specification Limit

LSL = Lower Specification Limit

Here:

measures the closeness between process mean and USL.

measures the closeness between process mean and LSL.

accounts for change in process mean.

A 10 % improvement in Process Capability Indix would represent in real terms that there is an improvement of 10 % in the change in process mean.