Scientists have determined that an effective way to lower cholesterol is to eat three servings of whole grains every day. Studies have shown that the cholesterol levels of people who did so were significantly lower after six months than were those of people who did not, even though the cholesterol levels of the two groups had been the same before the studies began. Clearly, eating whole grains can have an appreciable effect on cholesterol levels.
The answer to which of the following questions, if true, would be most useful in evaluating the claim about whole grains above?