3-1s and 4-1s

These rules are violated when three or four consecutive results are:

• Greater than 1SD.
• On the same side of the mean.

The 3-1s and 4-1s rules have two applications:

• Within a control level (for example, all Level 1 control results).

  Violations within a control level indicate systematic bias in a single area of the method curve.

• Across control levels (for example, Level 1, 2, and 3 control results in combination).

  Violations across control levels indicate systematic error over a broader range of concentrations.

The following figures show a Levey-Jennings Chart with a data point violating the 3-1s rule within a run and a 4-1s rule across a run.