Dietitian: Eating fish can lower one' s cholesterol level. In a study of cholesterol levels and diet, two groups were studied. The first group ate a balanced diet including two servings of fish per week. The second group ate a very similar diet, but ate no fish. The first group showed lower cholesterol levels, on average, than the second group. The two groups had displayed similar average cholesterol levels prior to the study.
The statement 'It rules out an alternative explanation of the data collected in the study' most accurately describes the role played in the dietitian's argument by the claim that the two groups had displayed similar average cholesterol levels prior to the study.