Why some SPC control charts get used and others get abandoned

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A quality manager contrasted two control chart binders from the same process, one maintained for over a year and one abandoned after three weeks. The difference came down to whether the chart's limits were computed from the correct subgrouping, so alarms reflected real events. The article explains matching chart type to data, such as I-MR for single measurements and Xbar-R for small subgroups, and warns against mixing shifts, cavities, or trays within one subgroup. It recommends at least 25 baseline subgroups and a small default set of Nelson or Western Electric rules, documented for audit. Each chart should also have a named owner and a clear response plan for when it alarms.

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