Process capability in industry: Setting preliminary statistical specification limits

摘要

Specification limits are required for products to ensure functionality. When a new process is started, customer requirements may not be fully known. At this point properties of the products are measured after production. Because customer requirements are not available at this stage, preliminary specifications are drawn based on statistical considerations. This article focuses on the problem of setting preliminary specification limits based on acceptable process performance. The statistic considered is the capability index C_(pk) where performance of a process is not acceptable if C_(pk) is below 1, and acceptable if it is more than 1.6667. An earlier study proposed a specification limit C_p that is related to only width, but not location. This article proposes specification limits that are related to the properties of C_(pk) itself. The interval is expressed as (x - ks, x + ks) for some value of k to be determined. The definition of k proposed here is different from one proposed for a tolerance interval. In this study it is assumed that the observations follow normal distribution or this can be achieved by a suitable transformation.