There are different ways to do this depending on which control chart platform you are running: With the Analyze>Quality and Process>Control Chart>Xbar Control Chart, simply change the subgroup size in the Constant Subgroup Size box (default is 5)
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In the Analyze>Quality and Process>Legacy Control Charts>Xbar, there is a similar option Sample Size Constant:
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"All models are wrong, some are useful" G.E.P. Box