Cpk
Actual process capability index
Like Cp, but it accounts for the shift of the mean from the centre of the tolerance. This is the index the customer cares about.
Formula
Cpk = min(USL − mean, mean − LSL) ÷ 3σ
A process can have a small spread and still produce rejects because it is set near the limit. Cpk shows that, Cp does not.
Typical requirements: Cpk of at least 1.33 for regular characteristics and 1.67 for critical ones. With few measurements the index is uncertain and not worth building conclusions on.
Example
A mean of 6.21 g with a tolerance of 6 ±0.3 g and σ = 0.043 gives Cpk = 0.09 ÷ 0.129 = 0.70. The process is not capable, although the spread is small.
What it looks like in the system
The capability board shows the Cpk of every characteristic with a colour. Broken down by mould cavity you see which cavity spoils the result.
Related terms