Three-dimensional variance dispersion graphs for mixture-process experiments

In a mixture experiment, the design factors are the proportions of the components of a mixture, and the response variables depend only on these component proportions. In addition to the mixture components, the experimenter may be interested in other variables that can be varied independently of one another and of the mixture components. Such mixture-process experiments are common in industry. There are many strategies based on different design criteria that are used to create designs involving both types of variables. We develop variance dispersion graphs (VDGs) to evaluate mixture-process designs and illustrate how the graphs are used with two examples.