A clinical psychologist is interested in determining the effectiveness of three different forms of therapy in treating clients with depression. Participants with the diagnosis are randomly assigned to one of the three treatments. They are given the Beck Depression Inventory (BDI) both before and after treatment. Pre-treatment BDI scores are used as the covariate. Which statistical procedure is used?

a. independent sample t test

b. ancova

c. chi square

d. multiple regression

e. manova