Control charts III: SOP 20 and the standard deviation chart

The mean chart of SOPs 9 and 17 watches whether a process's CENTER moves. SOP 20 completes the series with the chart that watches its SPREAD: plotting the standard deviation of each measurement group against limits derived from the pooled historical value.

The two charts fail differently, and that difference is diagnostic. A shifted mean with stable spread suggests a systematic change — a damaged standard, a new offset. A stable mean with growing spread suggests degradation — a failing bearing in a balance, a loosening connection, an operator improvising. Watching only the mean chart lets the second class of failure hide inside widening scatter until it finally lands a point out of limits.

The spread chart also guards the uncertainty claim itself: the budget's Type A line assumes the process standard deviation is what history says it is, and SOP 20 is the ongoing proof. All three chart procedures sit adjacent in the library — the complete measurement-assurance statistics toolkit in perhaps thirty pages.