Setting the control limits and the central line in X-chart

The control limits can be set based on method performance – statistical control limits or according to the requirement on within-laboratory reproducibility – target control limits.

Statistical control limits	Target control limits
The control limits are calculated from the historical results obtained during the analysis of the control sample. For at least 12 analysis carried out during a certain time period, e.g. a month, the standard deviation (s) of the results is calculated. This population of results must be free of out-of-control results. Warning limits shall be set at +2s and – 2s. Action limits shall be set at +3s and – 3s. Note: The statistical control limits are constantly updated with the more recent results under statistical control.	The control limits are set based on the requirement on the analytical quality. The standard deviation for the control chart, s, is estimated from the requirement on reproducibility. Warning limits shall be set at +2s and – 2s. Action limits shall be set at +3s and – 3s.

Statistical control limits

Target control limits

The control limits are calculated from the historical results obtained during the analysis of the control sample. For at least 12 analysis carried out during a certain time period, e.g. a month, the standard deviation (s) of the results is calculated. This population of results must be free of out-of-control results.

Warning limits shall be set at +2s and – 2s.

Action limits shall be set at +3s and – 3s.

Note: The statistical control limits are constantly updated with the more recent results under statistical control.

The control limits are set based on the requirement on the analytical quality. The standard deviation for the control chart, s, is estimated from the requirement on reproducibility.

Warning limits shall be set at +2s and – 2s.

Action limits shall be set at +3s and – 3s.

The central line in the control chart can be the calculated mean value of the control values or a reference value for the control sample. In most cases a mean central line is used.

Mean central line	Reference central line
The mean value is estimated from control values obtained during a longer time. The central line is set to this mean value.	The control sample is a reference material or a well-characterised material. The central line is set to the assigned (reference or certified) value.