Analysis of Covariance

Abstract

The analysis of covariance, or ANCOVA, technique is an amalgam of the ANOVA and regression techniques. In analysis of covariance we fit parallel straight lines to approximate the relationship between two variables, such as fatness and weight, for several groups, such as male and female sheep. This is usually done for one of two reasons. Firstly, a researcher may simply wish to describe the relationships. Alternatively, a researcher may be trying to increase the precision of comparisons between groups by explaining some of the variation in the y variable, say fatness, using a relatedxvariable, say weight, whichcovarieswithy. To rephrase this, we may be primarily interested in accounting for some of the extraneous variation which may affect the precision of a study, or bias the estimates of population means.