Analysis of Covariance

Overview: In experimental methods, a central tenet of establi shing significant relationships has to do with the notion of random assignment. Random assignment solves a couple of problems. Statistically, it ensures that, in the main, the resulting probabi lity will be independent of the starting conditions of an experiment. Secondly, it is a way of establi shing parity, which is to say that it is a method of controlling for what you don’t know. ANCOVA is a method that can be thought of as a cross between ANOVA and Regression. It is, in fact, an A NOVA where the effects of some other variable have been controlled for statistically. T here are three reasons why one would choose to use an ANCOVA: 1. Reduction of within group or error variance – to increase the sensitivity of a test of main effects and/or interactions, by reducing the error term. 2. Elimination of systematic bias – to adjust the means on th e d.v. to what they would be if all subjects scored equally on the c.v. 3. Stepdown Analysis – to compare scores on a d.v. after they are adjusted for scores on another DV, which is treated as a c.v. (MANOVA model).