Design and application of integrated control charts for monitoring process mean and variance

This article presents an integrated control chart system for monitoring process shifts in mean and variance in a multi-stage manufacturing system. This chart system consists of several X & S charts, each of which monitors one of the critical quality characteristics (for example, dimensions) of a product. The design algorithm optimally allocates the detection power of the system among different stages as well as between the X chart and S chart within each stage. Consequently, the performance characteristics of the system as a whole can be considerably improved and the product quality will be further enhanced. Such improvement is achieved without requiring additional cost and effort for inspection. Furthermore, floor operators can implement and understand the integrated chart system as easily as the conventional 3-sigma system. Some useful guidelines have also been brought forth to aid designers in adjusting the control limits of the charts in a system.