A survey was taken of students at a large university in the fall of 2003; among other things, students were asked to report their weight and how much money they had earned in the previous year. When earnings were regressed on weights, the correlation was found to be 0.19, suggesting that heavier students tended to earn more. However, there may be confounding variables entering in.
a. Which of these may be tied in with a student's weight and also have an impact on earnings: gender, age, both of these, or neither of these?
b. If gender is a suspected confounding variable, how should the regression be handled?