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The R-4s rule identifies random error. It is applied only within the current run. This rule is violated when there is at least a 4SD difference between control values within a single run.
For example, assume both Level 1 and Level 2 have been tested within the current run. Level 1 is +2.8SD above the mean and Level 2 is -1.3SD below the mean. The total difference between the two control levels is greater than 4SD (+2.8SD - (-1.3SD) = 4.1SD).
The following figure shows a Levey-Jennings Chart with two points violating the R-4s rule.
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