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 skewness indices.
Refer to the following for more information:
Upper Specification Limit and Lower Specification Limit
Upper specification limit (USL) and lower specification limit (LSL) specify the requirements of the product, and so are customer driven. Make the target value the midpoint between USL and LSL.
Cp Index
The inherent process capability (Cp) is the ratio of tolerance to 6-sigma. Essentially this index indicates whether the distribution would fit inside the USL and LSL limits. Its meaning is defined as follows:
Cpk Index
The process capability based on the worst case (Cpk) is similar Cp. This index, however, indicates where the mean lies in relation to the USL and LSL limits. It is used to mathematically clarify Cp. Its meaning is defined as follows:
Published June 2018