Laboratory Quality Control Practice Test

A single control measurement that falls more than three standard deviations away from the target mean triggers this rule. In a process that is in control, most control values lie within ±3σ of the mean, so hitting beyond that boundary is unlikely due to normal random variation (about 0.3%). That makes it a red flag for a potential issue—instrument drift, calibration problems, reagent or sample issues, or other errors that should be investigated.

Other patterns correspond to different rules: two consecutive values beyond ±2σ point to a potential shift or bias (a different rule), ten consecutive measurements on one side indicate a persistent bias, and a single value beyond ±2σ is not what this rule flags.