Which Westgard rule is triggered by a single control value exceeding the mean by more than 2 standard deviations?

• A. 1(2s) Westgard Rules
• B. 2(2s) Westgard Rules
• C. 1(3s) Westgard Rules
• D. 10x Westgard Rules

Answer: (A)

This question tests how Westgard rules respond to how a single control result lies relative to the expected distribution. If a single control value is more than two standard deviations above the mean, it falls outside the usual 95% range and signals a potential issue with the run. The rule that flags this exact situation is the one that triggers when a single point exceeds 2 SD from the mean, regardless of direction. This is different from rules that require two consecutive out-of-range points, or a single point beyond 3 SD, or ten consecutive points on the same side. So a lone value above 2 SD matches this rule's criterion, and it should flag the run.