ISOLATION OF DEGREES OF FREEDOM FOR BOX-BEHNKEN DESIGNS

The Box-Behnken three-level designs have proved useful for fitting sec ond-order response surfaces to experimental data. Here, we discuss an aspect of their value in the detection of lack of model fit. The degre es of freedom in excess of those needed to fit the model can be used a dvantageously for contrasts which estimate certain higher-order depart ures from the model. These can be expressed in various forms. Some for ms, in which these contrasts are orthogonal, permit the plotting of th e contrasts in a normal plot for easy detection of model bias.