Analysis of covariance is used to test the main and interaction effects of categorical variables on a continuous dependent variable, controlling for the effects of selected other continuous variables which covary with the dependent. The control variable is called the "covariate." There may be more than one covariate. One may also perform planned comparison or post hoc comparisons to see which values of a factor contribute most to the explanation of the dependent. See chapter 8 in Lehtonen, R & Pahkinen, E. (2004)

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