Use control-chart thinking to distinguish common-cause variation from special-cause signals.
A control chart is a discipline against emotional knob turning. Common Cause Is System Behavior Common-cause variation is built into the current process. It may still be unacceptable, but it will not be solved by blaming the latest operator or adjusting after every reading. It calls for system improvement: better materials, fixture control, maintenance, method, or environment. Special Cause Is a Signal Special-cause variation is evidence that something changed. A point beyond a control limit, a long run on one side, or a clear trend can justify investigation. The team should ask what changed in material, machine, method, measurement, people,…
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