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.
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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 .