Analysis of Covariance (ANCOVA) can be used to compare mean differences among groups with adjustment on covariates. Originally, ANCOVA was developed as a method for increasing the precision of estimated treatment effects in randomized experiments. For example, is there a mean difference between a treated group and a control group on the post-test (the dependent variable) with adjustment on the difference in pre-test (the covariate)? Over the years, ANCOVA has been used as a method of adjusting for group differences in observational studies. For instance, do smokers and non-smokers differ in health status indicators after adjustment for age and income? Adjustment means the effects of differences in participants age and income have been controlled.