In a classic experiment carried out by the biologist hopkins in the 20th century, 20 young rats were divided into two groups. one group was fed on a diet of pure carbohydrate, lipid and protein; the other was fed the same diet, but with some added milk. the rats were weighed every day. after 18 days the diets of the groups were swapped over.
why did hopkins use 2 sets of rats?
suggest some important control variables that hopkins kept constant during his investigation.