A unifying approach to process capability indices

Much controversy surrounds the use of process capability indices in quality assurance programs. Some of the controversies include the indices legitimacy and consistency in monitoring improvement, their abuse by practitioners in using estimates as parameters, and the appropriateness of an index in particular applications. Through the use of a weight function, a unified approach for some common indices is presented, and several properties associated with the various indices are examined. Elucidation of the relationships and statistical properties associated with the various indices may assist in reducing some of the controversies that currently exist.