The formula for Cpk (Process Capability Index) is used to determine how well a process is capable of producing output within specification limits. It provides a measure of the process's centering and spread relative to these limits. According to the reference, here's the formula:
Cpk = min ((USL-mean)/(3σ), (mean-LSL)/(3σ))
Where:
- USL = Upper Specification Limit
- LSL = Lower Specification Limit
- mean = The average of the process output
- σ = Standard deviation of the process
In simpler terms, Cpk takes the minimum of two values: how close the process mean is to the upper specification limit and how close the process mean is to the lower specification limit, both relative to the process variation (3σ). This ensures that Cpk reflects the worst case scenario, providing a conservative estimate of process capability.
