What Is Process Capability?
Process capability is a statistical measure of how well a process can produce output that falls within specification limits. It compares the spread of a process to the width of the specification tolerance. High capability means the process consistently produces output within acceptable limits with minimal defects.
Process capability indices are essential in manufacturing, Six Sigma, and quality management. Cp measures the potential capability (assuming perfect centering), while Cpk accounts for how well the process is actually centered between specification limits. A perfectly centered process has Cpk = Cp.
Cp and Cpk Formulas
Capability Index Interpretation
| Cpk Value | Sigma Level | Defect Rate (PPM) | Assessment |
|---|---|---|---|
| < 1.00 | < 3σ | > 2,700 | Not capable |
| 1.00 | 3σ | 2,700 | Barely capable |
| 1.33 | 4σ | 63 | Capable |
| 1.67 | 5σ | 0.57 | Very capable |
| 2.00 | 6σ | 0.002 | World class |
Frequently Asked Questions
What is the difference between Cp and Cpk?
Cp measures the potential capability of a process, assuming it is perfectly centered between the specification limits. Cpk accounts for the actual centering by using the distance from the mean to the nearest specification limit. If Cpk equals Cp, the process is perfectly centered. If Cpk is much less than Cp, the process is off-center and should be adjusted.
What Cpk value should I target?
A minimum Cpk of 1.33 is commonly required in manufacturing. Many industries require 1.67 for safety-critical applications. The automotive industry often requires Cpk of 1.67 for new processes and 1.33 for existing ones. For Six Sigma, the target is Cpk = 2.0.