Monitoring variance components

This article discusses methods for monitoring a process in which the variance of the measurements is attributed to several known sources of variability. For example, in the case of integrated circuit fabrication one is typically interested in monitoring the process mean as well as the lot-to-lot, wafer-to-wafer-within-lot, and within-wafer components of variability. The article discusses the problem of monitoring the process level and variability by using the cumulative sum technique. Some aspects of the implementation of this methodology are also considered, and examples are given. © 1994 American Statistical Association and the Amencan Society for Quality Control.