The Levey-Jennings chart was created in the 1950s to answer questions about the quality and consistency of measurement systems in the chemical and process industries. This column will illustrate the fatal flaw in this technique and show a better way to track the consistency of your measurement systems. In addition it will describe how to quantify the actual resolution of your measurements.

For our example we will use data from page 20 of Walter A. Shewhart’s Economic Control of Quality of Manufactured Product, first published in 1931. These data were collected as part of a research project on measuring the resistivity (in megohms) of an electrical insulator. Since this test was destructive, these measurements were made on samples cut from the same sheet of material. In this way the measurements were as close to multiple measurements of the same thing as destructive tests can be. The data are shown in figure 2. The average for these 64 values is 4430.4, and the global standard deviation statistic is 532.3.

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