Statistical FAQs
How are excluded rows handled by individual measurements and moving range control charts?
In JMP Versions 6.0, 6.0.1, and 6.0.2 when rows are marked to be excluded in a JMP data set, the excluded rows are still used in the computation of moving ranges for individual measurements and moving range charts (IR charts) in the Control Chart dialog. This is consistent with SAS, which provides the Exclude functionality for control charts via the data set switch variables _COMP_ and _COMP2_. _COMP_ and _COMP2_ are equal to ‘Y’ (Yes) for every row by default in SAS, meaning that every row is used in primary and secondary chart computations. Setting _COMP_=N for rows in the data set excludes subgroups from the computation of parameters and control limits in the primary chart. _COMP2_ works the same for secondary charts (MR, R and S). The important part is that this affects parameters and control limits, and moving ranges are not considered parameters. The process mean and standard deviation are parameters. So in SAS IR charts, if you exclude samples via _COMP_ and _COMP2_ then they are excluded from chart computations except moving ranges. For example, if the subgroup 2 value is used to compute the moving range for subgroup 3, setting the switch variable to N for the subgroup 2 row will prevent the subgroup 2 moving range from being used to compute the average moving range parameter, but the moving range for subgroup 2 is used and subgroup 2 appears in the moving range chart. In JMP, if subgroup 2 is excluded in this example then it will still be plotted in the moving range chart.
If you want excluded rows to be the equivalent of deleted rows when producing an IR chart, you will need to create a subset data set with those rows omitted. JMP Version 6.0.3 and above added a preference to control how excluded rows are handled for moving range charts. The default setting is unchecked the default behavior is to treat excluded points as if they were deleted (which is how previous versions of JMP (prior to JMP Version 6) have handled excluded points). Checking this box causes excluded points to be included in moving range charts, which is how SAS handles excluded points.
FAQ # 2112
Last Updated: 2007 June 29
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