Back to notes

SPC

Reading Cpk without false confidence

Cpk is a decision input, not a trophy. Stability, sample strategy, and special causes matter more than a single number.

2026-07-18 · 5 min read

A high Cpk on an unstable process is a misleading story. Confirm statistical control before treating capability as a guarantee.

Watch the window: short runs can look capable while seasonal or lot-to-lot shifts remain hidden.

Pair Cpk with control-chart state and process context (material, tooling, setup). Numbers without context create false confidence.

Use capability language that manufacturing leaders can act on: continue, investigate, contain.