The Westgard rules are a set of statistical patterns, each being unlikely to occur by random variability, thereby raising a suspicion of faulty accuracy or precision of the measurement system.
They are used for laboratory quality control, in "runs" consisting of measurements of multiple samples.
They are a set of modified Western Electric rules, developed by James Westgard and provided in his books and seminars on quality control.
[1] They are plotted on Levey–Jennings charts, wherein the X-axis shows each individual sample, and the Y-axis shows how much each one differs from the mean in terms of standard deviation (SD).
The rules are:[2] The recommended consequences when any of the above patterns occur is to reject the run, except for the rule of 12s (top in table), which serves as a warning and a recommendation of careful inspection of the data.