Abstract: A new class of capability indices, containing Cp, Cpk, Cpm, and Cpmk, is defined. By varying the parameters of the studied class, indices with different properties can be found. Two estimators of the indices are considered and, assuming that the studied characteristic of the process is normally distributed and that the target value is equal to the mid-point of a two-sided specification interval, their expected values, variances, and mean square errors are derived. It is shown that studying the properties of the class of indices alone, without taking the properties of its estimators into account, might be misleading.
Key words and phrases: Capability indices, estimation, expected values, mean square errors, variances.