Aresearcher is interested in studying changes in marital satisfaction over the course of marriage. she hypothesizes that marital satisfaction is highest when couples are newly married, that it decreases during the middle years of marriage, but rises again during later years of marriage. in order to test her hypothesis, she recruits fifty married couples within each of the following conditions: (1) married less than 10 years; (2) married 15-20 years; (3) married more than 25 years. all 150 couples (50 from each condition) complete a standardized measure of marital satisfaction. what is the dependent variable?