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Using statistics to evaluate QC values assumes new control measurements have a similar distribution to past measurements while the system is stable and the distribution of values is Gaussian (normal).
Using these assumptions, 95.5% of values should be within ±2SD of the mean and 99.7% of values should be within ±3SD of the mean. A value outside ±3SD of the mean would be expected only 0.3% of the time when the system is stable.
The Levey-Jennings Chart is helpful to visually identify these data points as well as shifts (sudden changes) and trends (gradual changes) in laboratory data.
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