One of two methods Minitab offers to calculate process capability statistics for nonnormal processes. The other option is the Minitab method. This method, recommended by the International Organization for Standardization, calculates capability statistics from the 0.135, 50.0, and 99.865 percentiles of the distribution you specify to model the data. The portion of the data distribution between the 0.135th and 99.865th percentiles corresponds to the 6s spread calculated in capability analysis for normal data.
The ISO method calculates capability statistics as follows:
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PPL = |
X0.5 - LSL |
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X0.5 - X0.00135 | |
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PPU = |
USL - X0.5 |
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X0.99865 - X0.5 | |
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Pp = |
USL-LSL |
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X0.99865 - X0.00135 |
where:
USL = Upper specification limit
LSL = Lower specification limit
X0.99865 = The 99.865th percentile of the distribution you specify to model the data
X0.00135 = The 0.135th percentile of the distribution you specify to model the data
X0.5 = The 50th percentile, or median, of the distribution you specify to model the data