You present results from a regression of weight on a dummy variable for a new weight loss drug. Someone criticizes the internal validity of the regression, claiming that gender is associated with both weight and taking the drug. What should you do to improve internal validity?
A. Control for gender in a multiple regression model
B. Interact the gender with a dummy for taking the drug
C. Restrict the sample only to men
D. None of the above
Answer: A. Control for gender in a multiple regression model