Basics of Six Sigma- Statistical Process Control

A: Shipping parts based on spec-only limits lets special-cause variation slip through until scrap or rework shows up downstream. Control limits come from the process, not the drawing, so they flag instability early and keep you from reacting too late.

A: Charting with a noisy gage hides real shifts and triggers false alarms, wasting hours and confidence. Proving the measurement system works keeps the signals tied to the process, not the tool.

A: Letting a trend run ends with parts drifting out of tolerance mid-shift and a scramble to contain product. Trend rules exist to catch predictable physical changes early, before the limits are crossed.

A: Poor subgrouping smears signals until a real shift looks like background noise, delaying action. Rational subgroups mirror how the process actually changes, so assignable causes show up fast.