Passage Summary: A dietitian argues that fish is heart-healthy because a study showed that people who added fish to their diet had better cholesterol than those who didn't, even though both groups started the study with the same average levels.

Conclusion: Eating fish can lower a person's cholesterol level.

Reasoning: A study compared two groups with similar diets, except one group ate fish and the other did not; the fish-eating group ended up with lower cholesterol despite both groups starting at the same level.

Analysis: The claim that the groups had similar cholesterol levels at the start is a crucial piece of evidence used to establish a baseline. By showing the groups were equal initially, the author strengthens the argument that the difference found at the end was caused by the fish rather than pre-existing conditions. In your analysis of the roles, identify this as a premise that helps rule out an alternative explanation for the study's results. It ensures we are comparing apples to apples—or in this case, salmon to salmon.