Product Engineering
A tolerance written without the process spread beside it is a wish, not a specification.
— Cp
The weaker of the two properties
Cp assumes the process is centred on nominal, which is the optimistic case — a process running off-centre has a lower Cpk than the Cp shown here, and correcting the centring is usually cheaper than reducing the spread. The process standard deviation must come from a stable, in-control process; computing capability from data that contains a special cause produces a number that describes nothing. Capability below about 1.0 means routine rejects are arithmetically guaranteed, and the honest responses are to widen the tolerance, improve the process or decline the specification — not to inspect harder.
Product Specification Limits & Process Capability Builder — free, with the formula and a worked example, at Textile School.