UNIVARIATE PROCESS CAPABILITY INDICES FOR UNILATERAL SPECIFICATION REGION — A REVIEW AND SOME MODIFICATIONS

In a manufacturing industry, often the quality characteristic under study is found to have only one of the two specification limits viz., upper specification limit (USL) or lower specification limit (LSL). In such cases the process capability indices (PCI) designed for bilateral specifications become inappropriate. However, in the literature only a few indices are available to address this problem. In the present paper, we have made an extensive study of the PCI's for unilateral specifications with a brief discussion of their possible fields of applications and drawbacks, if any. We have also proposed a logical formulation of a parameter of one of these indices which reduces the subjectivity of the index and hence makes it more suitable for practical application. An example, with the computed values of the various PCI's, is discussed to make a comparative study of the performance and inter-relationship between these PCI's. We have concluded the paper with a discussion on the future scope of study in this field.

DEVELOPMENT OF THE OPTIMUM PRODUCT SPECIFICATION FOR AN EXPONENTIAL-TYPE QUALITY CHARACTERISTIC

This paper investigates the problem of determining the most economical product specification (upper specification limit) for an exponential-type quality characteristic using the concept of a truncated quadratic loss function within the framework of a screening inspection scheme. Implementing a tight limit on products may provide a high outgoing quality, but it creates a high rejection cost on the rejected products. In contrast, implementing a loose limit may reduce the rejection cost, but it can result in a low outgoing quality. An immediate problem is then how to trade off these conflicting cost criteria to determine the most economical upper specification limit. This important issue has neither been satisfactorily addressed or received much attention in the literature. This paper presents the concept of the truncated quadratic loss function, applies the concept to screening inspection problems, derives the truncated statistics of interest, and presents the optimization model of the most economical upper specification limit for an exponential-type quality characteristic. A numerical example is presented.