Studies have shown that students who go to highly selective colleges have, on average, higher lifetime incomes than do students who go to less selective colleges.

a. Explain why there might be sampling bias in this study.

b. Suppose that the college you go to has no effect on your lifetime income, but that highly selective colleges are more attractive because they have much better dormitories, food, and extracurricular activities. Explain how it could still be the case that students who go to highly selective colleges have, on average, higher lifetime incomes than do students who go to less selective colleges.

c. What would have to be done for a controlled experiment?