Ppk

The capability indices of Ppk and Cpk use the mean and standard deviation to estimate probability. A target value from historical performance or the customer can be used to estimate the Cpm.

That spread being centered around the midpoint is part of the Cpk and Ppk calculations.

The midpoint = (USL-LSL) / 2

The addition of "k" quantifies the amount of which a distribution is centered. A perfectly centered process where the mean is the same as the midpoint will have a "k" value of 1.

Pp and Ppk use an estimate for sigma that takes into account all or total process variation including special causes (should they exist) and this estimate of sigma is the sample standard deviation, s, applies to most all situations. This estimation accounts for "within subgroup" and "between subgroup" variation.

The overall process performance indices, Pp and Ppk, most often use the sample standard deviation, s, formula as an estimate for sigma. There are other methods available for estimating the overall (total) process sigma.

The Cpk and Ppk will require two calculations, selecting the mininum is the value use as baseline and to compare to customer acceptability level. These can be calculated using unilateral or bilateral tolerances. Shown in the table below is the formula for bilateral tolerances where a LSL and USL are provided. If only one specification is provided (unilateral) then the value used for Cpk and Ppk is provided by the calculation that involves the specfication limit provided.

Pp and Ppk are rarely used compared to Cp and Cpk. They should only be used as relative comparisons to their counterparts.

Capability indices, Cp and Cpk, should be compared to one another to assess the differences over a period of time. The goal is to have a high Cp, and get the process centered so the Cpk increases and approaches Cp. The same applies for Pp and Ppk.

Cpk and Ppk account for centering of the process among the midpoint of the specifications. However, this performance indice may not be optimal if the customer wants another point as the target other than the midpoint. The calculation of Cpm accounts for the addition of a target value.