Which Westgard rule indicates systematic error manifested as a consistent shift across controls?

• A. 2-2s
• B. 1-3s
• C. 4-1s
• D. 1-2s

Answer: (A)

Systematic error shows up as a persistent bias in the process, a shift in the mean that appears across successive control measurements. When two consecutive control results both exceed the same 2-standard-deviation boundary on the same side, that pattern is unlikely due to random fluctuation and indicates the process has shifted. This is why this rule is the go-to for detecting a consistent bias across controls. In contrast, an isolated outlier beyond 3 SD points to a random spike, four consecutive results beyond 1 SD on the same side suggest a longer drifting trend, and a single result beyond 2 SD on either side is only a warning of possible issue. The two-in-a-row pattern specifically flags a true, repeatable shift in the process, i.e., systematic error.