Process capability measures how well a process performs relative to its specification limits. Cp tells you the potential capability if the process were perfectly centred; Cpk tells you the actual capability accounting for any shift off-centre. Both are calculated in the Measure or Analyse phase of DMAIC.
| Metric | What it measures | Accounts for centring? |
|---|---|---|
| Cp | The potential capability if the process were perfectly centred between specification limits | No — assumes the process mean is on target |
| Cpk | The actual capability, accounting for how far the process mean has shifted from target | Yes — penalises for being off-centre |
If Cp and Cpk are close in value, the process is well-centred. If they differ significantly, the process is off-centre even if it is technically within specification.
A Cpk of 1.33 is the minimum target for most processes, meaning the process spread fits within specification limits with a reasonable safety margin. A Cpk of 1.67 is the target for safety-critical processes. Below 1.0, the process is not capable and defects are likely.
Cp measures potential capability assuming the process is perfectly centred between specification limits. Cpk measures actual capability accounting for how far the process mean has shifted from target. If Cp and Cpk are close, the process is well-centred. If they differ significantly, the process is off-centre.
Yes. A negative Cpk means the process mean is outside the specification limits entirely — the process is producing defects on one side. A Cpk of 0 means the process mean is exactly on a specification limit. Both situations require immediate corrective action.
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