Control Charts: Do Not Chase Common Cause Noise
Interpret a control chart signal before changing a manufacturing process.
A bore diameter point is below nominal, but the control chart shows no point outside control limits and no nonrandom pattern. Control charts separate common cause variation from special cause signals so teams react to evidence, not anxiety. The common trap is tampering: adjusting a stable process because one point is not exactly where you hoped it would be. Check limits Confirm the sample point is inside upper and lower control limits. Specification limits and control limits answer different questions; the control chart asks whether the process has changed. Check patterns Look for runs, trends, cycles, or clusters that violate…
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