What Are Control Limits?

Control limit refers to the control range specified in the implementation of quality control procedures for analytical testing. The scope of control specified when implementing quality control procedures for analytical testing. In environmental monitoring, generally 99.7% of the total test results fall within the scope of quality control, that is, the control limit. ^[1]

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A control chart is a graphic record of the quality of the production process or process. There must be a centerline and a control limit on the chart. Some control charts also have a warning limit. The chart describes the statistical value of each sample drawn in chronological order. .

The center line is the average value of the statistic being controlled, and is the estimated value of the statistic's mathematical expectation or its expected value.

The control limit is a criterion for judging whether there is an abnormal cause in the production process or process. In the analysis control chart, at least one of 25 consecutive points is outside the limit or at least two of the continuous 35 points are outside the limit or at least three of the continuous 100 points Outside the limit, it is considered that there is an abnormal cause. As long as there is a point above or outside the limit in the control and utilization control chart, it is considered that there is an abnormal cause, and measures should be taken, so it is also called the limit of action.

There are three types of measures: investigating the source of the abnormal cause;

Assuming that the average value of all test results is X and the standard deviation is S, the statistical calculation is performed, and the control limit is X ± 3S. If an analytical result of a controlled QC sample falls outside the control limits. It means that the analysis results are out of control. ^[1]