Passage Summary: A doctor noticed that little kids who use night-lights are often nearsighted, but older kids aren't. He thinks this means the eye problems caused by the lights just get better over time.

Conclusion: If night-lights cause nearsightedness, the effect likely disappears as children age.

Reasoning: A study of younger children showed a link between night-lights and nearsightedness, while studies of older children showed no such link.

Analysis: The doctor assumes the difference between the studies is due to the age of the participants rather than a flaw in the first study or an outside factor. To weaken this, we should look for an alternative explanation for why the first study found a correlation while the others didn't. It is possible that the parents in the first study were nearsighted themselves and passed it on genetically, while also being more likely to use night-lights for their kids.