Three capability indices answer slightly different questions about your process: Cp measures potential capability (spread only), Cpk measures actual capability (centering + spread), and Cpm penalizes deviation from target.
| Index | Cp = Cpk? | What it means |
|---|---|---|
| Cp > 1, Cpk = Cp | Yes | Process centered and capable |
| Cp > 1, Cpk < Cp | No | Capable spread but off-center — adjust mean |
| Cp = Cpk < 1 | Yes | Process centered but too wide — reduce variation |
Cpm is used when a customer defines a target T that differs from the spec midpoint. A process exactly on spec midpoint may still have a low Cpm if the midpoint is not the target.
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