One of two methods Minitab offers to calculate process capability statistics for nonnormal processes. The other option is the ISO method. If you specify the Minitab method, it is automatically implemented as follows:
1 Minitab calculates the proportion of observations that are less than the lower specification limit and the proportion of observations that are greater than the upper specification limit.
2 Minitab calculates the inverse CDFs of these two proportions on the standard normal distribution, and designates them Z.LSL and Z.USL, respectively.
3 Minitab calculates the capability statistics as follows:
Pp = (Z.USL - Z.LSL) / 6
PPL = -Z.LSL / 3
PPU = Z.USL / 3
Ppk = Min {PPL, PPU}
A disadvantage of this method is that it cannot calculate these indices if the upper or lower specification limits are beyond the range of the distribution you specified to model the data.