Nelson rules

I’ve started getting into heuristics lately and while rabbit-holing through Pinboard, I found a method called “Nelson rules”:

Nelson rules are a method in process control of determining whether some measured variable is out of control (unpredictable versus consistent). Rules for detecting “out-of-control” or non-random conditions were first postulated by Walter A. Shewhart in the 1920s […]

The rules are applied to a control chart on which the magnitude of some variable is plotted against time. The rules are based on the mean value and the standard deviation of the samples.

There are 8 simple rules that focus on points in a chart and how they might deviate or remain close to the mean. However, as it is a heuristic, it’s not supposed to be taken as gospel but rather a yellow flag and calls for further observation.