Missing Data for Control Charts that use Weighted Statistics
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If a single observation is missing and you have data in subgroups, it is omitted from the calculations of the summary statistics for the subgroup it was in. All formulas are adjusted accordingly. This may cause the EWMA chart to produce control chart limits that are not straight lines.

Suppose an entire subgroup is missing:

·    EWMA plots an exponentially weighted moving average of all past subgroup means. The chart cannot be created if an entire subgroup is entered as missing

·    CUSUM plots a cumulative sum of deviations. Unlike the EWMA, when the CUSUM chart encounters a missing subgroup, the chart can be created but it leaves a gap for the missing subgroup.

·    Zone leaves a gap in the chart corresponding to the missing subgroup.

EWMA and Zone charts work with equal or unequal-size subgroups, but CUSUM charts require all subgroups to be the same size.