Ken is interested in determining if there are differences between men's occurrence of obesity and prior football team experience. He compares three different groups of men. In one group, he has men who played football in junior high but not in high school or after high school. Those in the second group played in junior high and high school but not after high school. Those in the third group played in all three. As part of the study, he weighs the men to calculate obesity. What type of research method is Ken employing?
A. Survey
B. Causal-comparative
C. Experimental
D. Correlational