Quality control charts are used to record the results of measurements on control samples, to determine if there are systematic or random errors in the method being used. The most common type of chart is the Levey-Jennings chart.
There should be a separate control chart for each method being monitored, and separate charts for normal and abnormal controls. The mean and standard deviation of the control being used should be noted on the chart. These should be determined based on at least 20 measurements over 20 days.
Here is an example of a Levey-Jennings chart:
Each time the control is tested, the result is marked on the chart at the appropriate standard deviation level. For instance, if the mean for a control is 15 and the standard deviation 5, if you test a control and get a value of 22.5, the chart is marked at +1.5 SD for that day.