How should the subgroup size be selected for an X-bar chart?

The key is to specify a subgroup size so that significant shifts are detected with high probability and that insignificant shifts are unlikely to produce a signal. Steven Wachs Principal Statistician Integral Concepts, Inc. (Note: Part II of this article will be published in DataNet Quality System's November newsletter.) The purpose of control charts is to detect significant process changes when they occur.

In general, charts that display averages of data/measurements (X-bar charts) are more useful than charts of individual data points or measurements. Charts of individuals are not nearly as sensitive as charts of averages at detecting process changes quickly. X-bar charts are far superior at detecting process shifts in a timely manner, and the subgroup size is a crucial element in ensuring that appropriate chart signals are produced.

Often, the subgroup size is selected without much thought. A subgroup size of 5 seems to be a common choice. If the subgroup size is not large enough, ... more.

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