The event "One control value exceeds the mean ±3SD" corresponds to which Westgard rule?

• A. 2(2s) Westgard Rules
• B. 1(3s) Westgard Rules
• C. 10x Westgard Rules
• D. R(4s) Westgard Rules

Answer: (B)

In quality control, a single data point that falls outside three standard deviations from the mean is treated as a red flag for a potential gross error. The rule that matches this scenario says that any one control value beyond the ±3 SD limit should cause the run to be rejected or investigate, because such a lone outlier is unlikely to occur by chance in a stable process. This rule is meant to catch occasional, large mistakes—like a calibration hiccup or an instrument issue—before reporting results.

Other Westgard rules require patterns across multiple measurements to signal a problem: for example, two consecutive points beyond ±2 SD on the same side suggest drift or a shift; ten consecutive points on the same side indicate persistent bias; and a run where the range between the highest and lowest control values exceeds a certain threshold signals a broader issue. But a single value outside the ±3 SD limit specifically corresponds to the rule that flags that lone outlier.