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 claim that the two groups had displayed similar average cholesterol levels prior to the study provides background information on the purpose of the study.