A Cpk value is a statistic calculated using a model that is fitted to sample data (quantity N). This statistic is a “point estimate” (Ĉpk) with an associated Confidence Interval. Given a large number of Cpk point estimates from an in-control process, the true value of Process Capability (Cpk) is expected to be within the Confidence Interval for (1-)*100% of those estimates, i.e. for 95% if = 0.05, where is the “significance” and 1- is the “confidence.” The Confidence Interval depends on (taken as 0.05), N, and Ĉpk itself