ANCOVA is a procedure like ANOVA except researchers can study the effects of one or more independent variables on a dependent variable after adjusting for other variables, called covariates, which were not a primary focus of the study. The letter C in ANCOVA stands for covariate. There can be several covariates in a study. In testing for differences among groups experiencing different leadership styles, we could study the effects on employee satisfaction after adjusting for a covariate of years of employment. A key word in ANCOVA studies is adjusting. Analysts adjust the scores based on information about the covariate before testing for significant differences.
Basic features of an ANCOVA:
Independent or grouping Variable = 1 or more
Dependent or criterion Variable = 1
Covariates = 1 or more
An F test indicates significance overall and for specific effects or relationships.
A commonly reported measure of effect size is eta squared.
A p value reveals the probability of a significant relationship-- one that is not due to chance factors.
Read more about ANCOVA in the following books.