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Physics1 min readJune 18, 2024
SPC on the Shop Floor: When Your Control Chart Lies
Moving averages, tampering and forgotten MSA — three reasons a chart can look in control while the process is not. A practical guide.
Control ≠ capability
A control chart without a qualified measurement system (MSA) is a broken thermometer.
MSA first
- %GRR <10% ideal, <30% tolerable
- Bias vs standard
- Resolution ≥5 distinct categories
Don’t over-average
Averaging 5 consecutive parts hides within-lot variation. Prefer Xbar-R/S for rational subgroups, XmR for individual readings.
Stop tampering
Adjusting the machine on every out-of-spec point amplifies variation (Deming’s funnel). Use Western Electric rules and a formal reaction plan.
Process physics explains variation; statistics makes it visible. One without the other is blind.
About the author
Charafeddine Barkat — Pharmacist, Technical Production Director & Physicist.
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