A Levy-Jennings chart is a graph that quality control data is plotted on to give a visual indication whether a laboratory test is working well. On the x-axis the date and time, or more usually the number of the control run, are plotted. A mark is made indicating how far off the actual result was from the mean (which is the expected value for the control). The distance from the mean is measured in standard deviations (SD). Lines run across the graph at the mean, as well as one, two and sometimes three standard deviations either side of the mean. This makes it easy to see how far off the result was. Rules, such as the Westgard rules can be applied to see whether the results from the samples when the control was done can be released, or if they need to be rerun. wikipedia.org