Analysis of Variance of Repeated Measures: Visual Acuity
Unfortunately, the advantages of a repeated measures design come at a cost, and that is the probable lack of independence of the repeatedmeasurements. Observations made under different conditions involving the same subjects will very likely be correlated rather than independent. This violates one of the assumptions of the analysis of variance procedures described in Chapters 4 and 5, and accounting for the dependence between observations in a repeated measures design requires some thought. In the visual acuity example, only within-subject factors occur, and it is possible, indeed likely, that the lens strengths under which a subject was observed were given in random order. But in examples, where time is the single within-subject factor, randomization is not, of course, an option. Thismakes the type of studywhere subjects are simply observed over time rather different from other repeated measures designs, and they are often given a different label, longitudinal designs. Due to their different nature we shall leave consideration of longitudinal data sets until Chapters 12-14.