Logical reasoning PrepTest 104 · Section 1 · Question 23

This answer messes up the stimulus. There, 90% of extreme insomniacs drink a lot of coffee, and 10% of them don’t. We don’t learn anything about the percentage of coffee fiends who are or are not extreme insomniacs.

The argument isn’t about causality, but the overlap between groups. As such, other causes of extreme insomnia are out of scope.

This answer highlights the reversal in the stimulus. While we learn that 90% of extreme insomniacs are coffee fiends, we don’t learn what percentage of coffee fiends are extreme insomniacs. In using Tom’s heavy coffee drinking to conclude something about the likelihood he has extreme insomnia, however, the argument assumes that the 90% applies to that group. So the argument is relying on evidence about the frequency of extreme insomnia among heavy coffee drinkers, but it presents evidence about the frequency of heavy coffee drinking among extreme insomniacs. This answer describes the error in reasoning in the stimulus, so it’s the correct answer.

This answer does describe flawed reasoning - applying a generalization to an individual. However, the conclusion is that Tom is quite likely an extreme insomniac, and if we knew that almost every coffee fiend had that condition, then the statistics would be in favor of drawing that conclusion. It’d be akin to saying that almost every human lives on Earth, and Tom is a human, so he very likely lives on Earth. Sure, he may be an astronaut on the space station, but the stats are on the side of the conclusion that it’s “very likely” he lives on the planet.

Since the argument mentions 90% and “quite likely,” this answer is too certain in talking about “always.” Also, it brings up causes which, as we looked at in answer choice (B), is out of scope here.