Likelihood Ratio Methods for Monitoring Parameters of a Nested Random Effect Model

Abstract

In many practical situations the variance of a set of measurements can be attributed to several known sources of variability. For example, if several measurements of each item of a lot are taken, then one may need to deal not only with the within-item variability, but also with item-to-item within-lot and lot-to-lot components of variability. In such cases conventional control charts tend to produce an unacceptably high rate of false alarms and in general represent a rather weak diagnostic tool. This article shows how to build a control system based on likelihood ratio tests capable of monitoring the mean and variance components of a nested random effect model. The strong points and weaknesses of this approach are compared to those of competing methods, and some examples related to manufacturing of integrated circuits are discussed.

Key Words:

• Change point
• Control chart
• Cumulative sum
• Quality control
• Run length
• Variance components