Journal of Quality Technology vol:38 issue:2 pages:162-179
In many industrial response surface experiments, some of the factors investigated are not reset independently. The resulting experimental design then is of the split-plot type, and the observations in the experiment are in many cases correlated. A proper analysis of the experimental data therefore is a mixed model analysis involving generalized least-squares estimation. Many people, however, analyze the data as if the experiment was completely randomized and estimate the model using ordinary least squares. The purposes of this article are to quantify the differences in conclusions reached from the two methods of analysis and to provide the reader with guidance for analyzing split-plot experiments in practice. The problem of determining the denominator degrees of freedom for significance tests in the mixed model analysis is discussed as well.