A comparative psychologist suspects that chemicals in the urine of male rats trigger an increase in the activity of other rats. To check this hunch, she randomly assigns rats, in equal numbers, to either a sterile cage, a cage sprayed with a trace of the chemicals, or a cage sprayed thoroughly with the chemicals. Furthermore, to check out the possibility that reactions might be sex-linked, equal numbers of female and male rats are assigned to the three cage conditions. An activity score is recorded for each rat during a 5-minute observation period in the specified cage.