Problem: A marketing class designs two videos advertising an expensive Mercedes sports car. They test the videos by asking fellow students to view both (in random order) and say which makes them more likely to buy the car. Mercedes should be reluctant to agree that the video favored in this study will sell more cars because
(a) the study used a matched pairs design instead of a completely randomized design.
(b) results from students may not generalize to the older and richer customers who might buy a Mercedes.
(c) this is an observational study, not an experiment.