Passage Summary: A researcher says stretching is useless for preventing injuries because a study found no difference in injury rates between people who stretch and those who don't.

Conclusion: Stretching before jogging does not help prevent injuries.

Reasoning: A study showed that a group of joggers who stretch and a group who do not stretch had roughly the same number of injuries.

Analysis: The researcher assumes the two groups were identical in every other way, but this is a classic correlation-causation gap. If the people who stretch were more prone to injury to begin with, then stretching might be the only thing keeping their injury rates as low as the 'healthy' group. To weaken this, look for an answer that suggests the groups were not comparable at the start of the study. If stretching was compensating for a pre-existing weakness, the researcher's conclusion falls apart.