Analysis of covariance combines some of the features of both regression and analysis of variance.
These statements produce Output 30.4.1 . This model assumes that the slopes relating posttreatment scores to pretreatment scores are parallel for all drugs. This extra test is omitted in this example, but it is insignificant, justifying the equal-slopes assumption. This measures the difference between arithmetic means of posttreatment scores for different drugs, disregarding the covariate. This measures the differences between Drug LS-means, controlling for the covariate.