Type III Error

A Control Chart (X-Bar-and-S Chart, the basic prototype of all Shewhart Control Charts) is a kind of open ended One-Way Analysis of Variance. Each new Subgroup is a new “Treatment.” If all of these subgroup treatments have the same population or process mean, then the process is “In Control.” If some of the subgroup treatments have different such means, then these are Special Causes and the process is “Out of Control.”

Usually the Special Causes are thought of as specific changes like Fixed Effects in Analysis of Variance. But much of the undercurrent of Special Causes is random. No one plans them or can predict them. If the Random Effects One Way Analysis of Variance is then the model for a Control Chart, then in effect we are dealing with Variance Components.

Variance Components is a good first step in analysis of a process before implementing a Control Chart.

posted by X-Bar @ 3:03 PM 0 comments