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 that played football in jr. high, but not high school or post-high school. Those in the second group played in jr. high and high school but not post-high school. Those in the third group played all three. As part of the study he weighs the men to calculate obesity. What type of research method is Ken employing?