Use a Fixed Mean and Fixed SD with SPC Rules

Most SPC rules evaluate data against a floating mean and standard deviation (SD). For example, the 1-3s rule evaluates if the data point is ±3SD from the mean.

Specify a fixed mean, fixed SD, or both for each test. Fixed statistics are more sensitive than floating statistics. However, over time, cumulative statistics stabilize and can simulate a fixed statistic. Consequently, cumulative statistics provide a reasonable degree of sensitivity to shifts and trends.