How to Calculate Process Capability
What is Process Capability?
Analyzes whether a process can consistently meet specifications. Compares process variation to specification limits.
Formula
Cpk = (specification - mean) ÷ (3 × std dev)
Step-by-Step Guide
- 1Measure process output (25+ samples)
- 2Calculate standard deviation
- 3Define specification limits
- 4Cpk = (specification - mean) ÷ (3 × std dev)
- 5Cpk >1.33 capable, >1.67 excellent
Worked Examples
Input
±0.1 spec
Result
Cpk 0.67
Common Mistakes to Avoid
- ✕Confusing Cp (potential) with Cpk (actual capability)
- ✕Using too few samples or not accounting for stratification
Frequently Asked Questions
What Cpk indicates capability?
Below 1.0 unable to meet spec; 1.0-1.33 capable with tight control; >1.67 excellent.
What causes low Cpk?
High variation, process drift, poor equipment, untrained operators, or unrealistic specifications.
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