A measure of how well the process or product measurements are able to stay within the defined limits or specifications over the long term. Capability indices include Cp, Cpk, Cpm, Pp and Ppk.

The calculations use the standard deviation of the measurements compared to the specification limits. If the specifications or tolerance limits are wider than the process or product measurements, then it is deemed “capable”.

The difference in the capability indices are based upon calculations of the standard deviation, incorporation of the mean (average), and alignment of the mean to the target or nominal value.

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