Individuals Charts and Additional Tests for Changes in Spread

Some authors recommend the use of an additional test for detecting increases in the spread, when using a control chart for individual observations. We examine this recommendation both in a practical situation and theoretically. Both studies show that the additional test gives somewhat more power for detecting a 25% increase of the process variation. For nearly all other deviations from the in-control state the test is more likely to cause confusion. From a practical viewpoint we therefore advise against its use. ∗ Dr. Trip is a senior statistical consultant in the Institute for Business and Industrial Statistics (IBIS) of the University of Amsterdam. Email: [email protected] † Dr. Wieringa is an Assistant Professor in the Faculty of Economics of the University of Groningen. Email: [email protected]