control limits. The handbook provides additional guidelines for control charts where the control limits are not symmetrical, as for R charts and p-charts. For and R charts (which plot the behavior of the subgroup range), the Handbook recommends using the zone rules above for subgroups of sufficient size (five or more). For small sample subgroups, the Handbook recommends: For other control charts based on skewed distributions, the Handbook recommends: Finding the probabilities associated with each zone Devising tests that should result in the desired sensitivity to the presence of assignable causes Other unnatural patterns The handbook also identifies patterns that require consideration of both the upper and lower halves of the control chart together for identification: See also Nelson rules Westgard rules References Quality control tools Technical communication Statistical charts and diagrams