Process capability index

In process improvement efforts, the process capability index or process capability ratio is a statistical measure of process capability: The ability of a process to produce output within specification limits. The concept of process capability only holds meaning for processes that are in a state of statistical control.
See more at Wikipedia.org...

the number of standard deviations between the process mean and the closest process specification limit divided by 3, abbreviation Cpk. For example, if there are 6 standard deviations between the mean and the nearest specification limit the process Cpk is 2.0. Generally speaking a Cpk >1.3 is considered capable (4 std deviations) and >2.0 is excellent.

The value of the tolerance specified for the characteristic divided by the process capability. The several types of process capability indexes include the widely used Cpk and Cp .