The residential division of Prism's high-speed Internet service uses one advertising agency, while its commercial division uses another. Two analysts, Andy and Brad, are asked to test the effectiveness of the two agencies. Andy proposes an A/B test that compares the click-through rates per ad of the two agencies. Brad proposes a difference-in-difference test in which the budgets for both agencies are increased by 50%, and the percentage changes in the click-through rates are compared. True or False: Both Andy's estimator and Brad's estimator are unbiased.