Butterfat is the fat that naturally occurs in cow’s milk. For a product to be labeled whole milk, for example, it must contain at least 3.25% butterfat. Does the amount of butterfat vary with different species of cows? Researchers gathered percent butterfat data on a sample of 100 cows. The sample of 100 cows consisted of 20 cows from each of 5 different species, where 10 in each species were considered mature and 10 were considered young. Why do you think they made sure there were 10 mature cows and 10 young cows for each species?