New Process Capability Indices for One-Sided Tolerances

Capability indices are dimensionless quantities measuring the ability of a process to manufacture items whose characteristics must be within a specified tolerance range. In this case and for a normally distributed process, the indices C(p), C(pk), C(pm), and C(pmk) are widely used. In this paper we study the case where a single tolerance is imposed because the shifts in the direction of this tolerance seem more serious than in the opposite direction. We propose a family of four indices having better properties than the existing indices. These new indices are created from the usual properties and interpretations of the indices C(p), C(pk), C(pm), and C(pmk), and can be used without difficulty in industry.