Economic Design of a Partially Dynamic •‐Control Chart

Abstract

In the economic design of •‐control charts the two extreme approaches used are that the design parameters are: (i) fixed once determined, and (ii) change from sample to sample. Administrative convenience favours the first approach in practice, although research work indicates, for instance, that constant sampling intervals are not necessarily optimal. This article provides an economic but partially dynamic model where the sampling intervals are of random lengths but other parameters are kept fixed once determined. Random sampling intervals are practical especially in labour‐intensive economies and job‐shop situations where assignable causes are introduced by external factors such as operator skill, fatigue, carelessness and other physical environments. In such work environments, operators have been found to take undue advantage of constant sampling intervals. The model developed here incorporates statistical, economic and administrative considerations in addition to other practical work situations. Some guidelines for implementing the model in the production environment are provided.