Missing Data with Data in Subgroups
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If a single observation is missing, it is omitted from the calculations of the summary statistics for the subgroup it is in. All formulas are adjusted accordingly. This may cause the control chart limits and the center line to have different values for that subgroup. This may also cause the Moving Average chart to produce control chart limits that are not straight lines.

If an entire subgroup is missing, there is a gap in the chart where the summary statistic for that subgroup would have been plotted. However, Moving Average charts treat missing data differently. Moving average charts plot a moving average of K subgroup averages. When the span of the moving average is three (default) and three or more consecutive samples are missing, there will be a gap in the chart corresponding to all of the moving averages that would have used that subgroup mean. You must have at least one sample within a span to calculate the moving average.