Confidence interval
range likely to contain the true value
A confidence interval gives a range around an estimate, such as a mean or a Cpk, in which the true value lies with a stated probability, usually 95 %. The fewer values you have, the wider the interval.
A Cpk of 1.50 from 20 values can have a lower bound below 1.0, so it does not prove capability.
Calculate it
Sample Size Calculator: Parts needed for a Cpk study, a mean, a defect rate or a zero-failure test.