What are the assumptions of ANCOVA?

Your data must meet the following assumptions for a one-way ANCOVA to give you a valid result:

1.     The dependent variable and covariate variables are measured on a continuous scale (at the interval or ratio level).

2.     The independent variable consists of two or more categorical, independent groups.

3.     There is independence of observations (there is no relationship between the observations in each group or between the groups themselves).

4.     There are no significant outliers or no points within data that do not follow the regular pattern. 

5.     The dependent variable is approximately normally distributed within each subpopulation. 

6.     There is homogeneity of variances (the variance of the dependent variable is equal over all subpopulations).

7.     The covariate is linearly related to the dependent variable at each level of the independent variable. 

8.     There is homogeneity of regression slopes (there is no interaction between the independent variable and the covariate).