Probability and the Westgard Rules

Shewhart introduced industry to the concept of three standard deviation (3S) limits for quality control and Levey, Jennings, Henry, and Westgard adapted this idea to clinical laboratory medicine. Westgard ultimately formulated a system of rules to enable clinical laboratory scientists to decide whether the tests they were doing were "in control" and reportable, or "out of control." The present communication explains mathematically how Shewhart's "beyond 3S" (ie, > 1 chance in 370) is an indication for corrective laboratory action.