N-x Rules

N_x rules are violated when there are 7, 8, 9, 10, or 12 control results on the same side of the mean.

Each of these rules has 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 7_x control rule is far more sensitive to analytical bias than the 12_x rule. The chance of finding seven consecutive control observations on one side of the mean is much higher than finding twelve.

The following figures show a Levey-Jennings Chart with data points violating the 8_x rule within a run and the 9_x rule across a run.

8_x Within a Control Level

9_x Across Control Levels