Maybe activity rates. One group could be sedentary, the other active in athletics. Perhaps researchers could have measured the cardiovascular strength of both groups. So this graph represents differences between Black Males and White Males. One Plausible Answer Gender is controlled .
Race is the independent variable.
Homicide rate is the dependent variable, which means that we are looking at how the homicide rate varies by race, over age groups in interims of five years. Look carefully at the graph to be sure you can see these relationships. A controlled variable is one that we choose to not measure. In this case we chose to look only at men. Had we looked at the homicide rate for women, too, we might have found greater balance between Blacks and Whites.