The process capability chart is a 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 sample data currently displayed (on the Mean chart). CitectSCADA automatically takes the data being trended, builds a distribution, adds the LSL and USL. It also calculates 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.
The process capability is defined in relation to the upper and lower specification limits (USL and LSL) for a given SPC Tag. These values are defined in SPC Tags and accurately represent the users requirements.