Control Charts for Small Lot Processes with Different Variances

Abstract

This article discusses about a practical problem associated with the insufficiency of observationsdata for constructing variables control charts in order to monitor the status of small lot processes withnon-constant variances in each lot. The mathematical model used to explain the variances of the processis also presented. The objectives of the paper are to propose an effective approach to dealing with theproblem by using standardized control charts and to illustrate the principles in using the chartingtechniques. Furthermore, the examinations and comparisons between the deviation-from-nominal controlchart and standardized control charts is performed by using a set of data from a case-study process. Theresult of the study points out that in practice applying the deviation-from-nominal chart to monitor smalllot processes with unequal variances should be prohibited. In this regard, it is recommended to implementthe standardized control charts instead.

Keywords : Variables Control Chart / Small Batch Process / Variance / Standardized ControlChart / Deviation-From-Nominal Control Chart