Which statement best describes the 2(2s) Westgard Rule?

• A. One control value exceeds mean ±2SD
• B. Two consecutive control values exceed mean ±3SD
• C. Two consecutive control values exceed mean ±2SD
• D. Ten consecutive on one side

Answer: (C)

Two consecutive control measurements beyond the same ±2 SD limit flags a potential shift or bias in the process. In this Westgard notation, the first number (2) means two successive points are required, and the (2s) means those points must lie beyond 2 standard deviations from the mean on the same side (both above or both below). This rule is designed to catch a gradual drift that wouldn’t be flagged by a single outlier, since random variation can produce one excursion, but two in a row in the same direction suggests a real change in the system.

For example, if the first QC value is above mean +2 SD and the next one is also above +2 SD, you would trigger this rule and investigate. If only one of the two is beyond ±2 SD, or if the two exceedances are on opposite sides, the rule is not violated.

Other options describe different rules: a single exceedance beyond ±2 SD isn’t what this rule uses, two consecutive results beyond ±3 SD correspond to a different rule, and ten consecutive results on one side refer to the 10x rule.