18.5. PRE-CONTROL
The PRE-Control method was originally developed by Dorian Shainin in the 1950s. According to Shainin, PRE-Control is a simple algorithm for controlling a process based on the tolerances. It assumes the process is producing product with a measurable and adjustable quality characteristic which varies according to some distribution. It makes no assumptions concerning the actual shape and stability of the distribution. Cautionary zones are designated just inside each tolerance extreme. A new process is qualified by taking consecutive samples of individual measurements until five in a row fall within the central zone before two in a row fall into the cautionary zones. To simplify the application, PRE-Control charts are often color-coded. ...
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