Marcucci, Mark O.; Beazley, Charles C. (1988, ASQC) General Foods Corporation, Tarrytown, NY
Capability indices such as Cp and Cpk are useful management tools for tracking process performance. They offer measures of a process characteristic standardized by the characteristic's target or specification. When the product characteristic is on target the process is operating at its economic optimum. When the product characteristic is within its designed specifications, the product meets the company-determined needs of the marketplace.
In this paper we discuss the uses, statistical properties, limitations, and interpretations of various capability indices for both variable and attribute data. For product characteristics which are continuous, the C-type indices focus on process process variation and deviation from target. An index we introduce, Cpg, also provides rough diagnostics of the sources of variation. The statistical properties of the Cp-type indices are reviewed, and the efficacies of the different indices are contrasted. The Cg-type indices are not suitable for measuring process performance for attribute characteristics. We propose an alternative index, Ca, for binomial characteristics which is based upon the odds-ratio. To decrease the variation inherent in estimators of characteristics with a small proportion defective, we use a shrinkage estimator of the standard odds-ratio. Some statistical properties of this index are discussed.
Chemical and process industries,Process capability studies