Daniel conducts a study looking at the relationship between exercise and depression and finds there is a negative correlation. Daniel concludes that exercise causes people to become less depressed

However, a colleague points out the possibility that less depressed people may simply be more likely to exercise. This represents a problem in interpreting correlations best known as ____________.
a. The problem of direction
b. The third variable problem
c. Experimenter bias
d. Confounding

Answer: a