A nutritional software program called DietPower offers encouragement to its users when they sign in each day. In one instance, the program states that people at their ideal body weight tend to have higher salaries than do people who are overweight and then explicitly states that losing weight might lead to an increase in pay!

a. Why is this a problematic statement? List at least two confounding variables that might affect this finding.

b. Imagine that you were going to conduct a study that compared the salaries of two groups: people who were overweight and people who were at their ideal body weight. Why would it be useful to include one or more covariates? What scale variables might you include as covariates?