Focus on the Classroom: Developing Control Charts and Illustrating Type I and Type II Errors

In this article, an entertaining class exercise that effectively illustrates the development of control charts, the impact of various levels of significance, and the two types of errors that can be made when using control charts is described. Actual sample data drawn from a sampling bowl in an in-class simulation are used to construct control charts for proportion defective. Then, how subsequent draws from the sampling bowl can result in either a correct decision that the process remains in control or a Type I error is illustrated. The proportion defective in the bowl is increased to simulate a process mean shift and illustrate how the new data can result in a correct decision that the process is out of control or a Type II error. This exercise also generates valuable insights about the trade-offs between significance levels and Type I and Type II errors.