Capability Charts

The process capability chart is the frequency histogramA bar graph that shows frequency of occurrence versus value. Quite often the data is fitted to a distribution such as a normal distribution. . and distribution of the process mean data and is used to analyze process capability. This chart is drawn with center line, lower specification limit and upper specification limit indicators in place. Interpreting capability charts is typically made with the Cp, Cpk, kurtosisAn index indicating the degree of peakedness of a frequency distribution (usually in relation to a normal distribution). Kurtosis < 3 indicates a thin distribution with a relatively high peak. Kurtosis > 3 indicates a distribution that is wide and flat topped. and skewnessAn index indicating the degree of asymmetry of a frequency distribution (usually in relation to a normal distribution). When a distribution is skewed to the left (for example), then the tail is extended on that side, and there is more data on the left side of the graph than would be expected from a normal distribution. Positive skew indicates the distribution's mean (and tail) is skewed to the right. Negative skew indicates the distribution's mean (and tail) is skewed to the left. indices.