After unsuccessfully searching Google on two occasions for the Wheeler rules for detecting an out of control process, I decided to publish them here so I could find them again in the future. According to Wheeler (as described in his book, Understanding Statistical Process Control) the four detection rules are:

1. A lack of control is indicated whenever a single point falls outside the three-sigma control limits.
2. A lack of control is indicated whenever at least two out of three successive values fall on the same side of, and more than two sigma units away from, the central line.
3. A lack of control is indicated whenever at least four out of five successive values fall on the same side of, and more than one sigma unit away from, the central line.
4. A lack of control is indicated whenever at least eight successive values fall on the same side of the central line.

Rules one and four are the first tests to run because they are easy. Rule 2 and 3 can be used when earlier detection is needed. These rules are designed to detect sustained shifts. Other run tests should be used to detect oscillation and changes in trends.

See you next time…