In May 1924, Walter Shewhart wrote a memo that contained the first example of a process behavior chart (i.e., a “control chart”). It was a chart for individual values that would be known today as a p-chart. Shewhart’s insight was that “three-sigma” limits will filter out virtually all of the routine variation so that points outside these limits are likely to represent process changes.

Following Aristotle’s lead, Shewhart realized that by studying the process changes we can discover their causes and gain insight on how to operate the process more consistently. To recognize the anniversary of Shewhart’s memo, this article outlines some of the history of the process behavior chart.

While Shewhart’s original chart was for proportions, he went on to develop process behavior charts for measurements by using a series of subgroups containing two or more original measurements. These charts plotted an average and dispersion statistic for each subgroup and placed limits of each of these running records. In practice, this quickly became the average and range chart we know today.

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