Abstract
In the first of two articles, we consider ways of obtaining additional information from control charts. The main purpose of the chart is to monitor a process or system, detect and diagnose problems and identify improvement opportunities. Some retrospective techniques (for completed charts) are presented here which yield more powerful diagnostics for establishing whether the process is stable and identifying whether any instability is random or systematic. All the methods can be implemented using hand-held scientific calculators with X, s functions, but can also be readily programmed for routine use on computer-based systems.
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