Fig. 1

Example control charts. a random variation. b Non-random variation caused by a large, possibly transient, shift in data identified by one data point being outside the upper control limit. c Non-random variation caused by a sustained moderate shift in data identified by an unusually long run of 13 data points below the centre line (Western Electric rule 4 and Anhoej rule 1) and unusually few crossing (Anhoej rule 2)