Abstract

A simple method for calculating process capability indexes is proposed. The method uses the Pearson family of curves and applies to any shape of distribution. Estimates of the mean, standard deviation, skewness, and kurtosis for the process are required to use this method, but the usual rules that define valid data for calculating process capability indexes apply. Use of Pearson distribution curves is explained, and a worksheet for the procedure is provided. Standardized values required in the calculations are tabulated, adapted from the book Non Normal Data Analysis.

Using Pearson distribution curves for calculating process capability indexes (Cpk) allows a graphical representation of non-normality by hand or using hand-held calculators. Includes charts of standardized tails of Pearson curves.