Normalizing Transformations for Shewhart-Type Control Schemes

Abstract

Traditional Shewhart-type process control schemes and process capability analyses assume that the process distribution (or distributions of statistics derived thereof) are approximately normally distributed. When this assumption fails to materialize, normalizing transformations are sometimes needed. However, many of the currently used transformations are difficult to apply and at times require expertise that the common practitioner does not possess. In this paper, a new set of normalizing transformations are suggested that are simple to carry out, may be generally applied (since they are distribution-free) and are associated with a non-iterative standard procedure that is easy to program. Implications for current practices regarding Shewhart-type control schemes and for capability analyses are discussed.