PrepTest 104
[lcid:3512] Prep Test 104 LSAT — Logical Reasoning — S1
Logical reasoning
Question prompt
Research indicates that 90
Remaining source text redacted.
Why the credited answer is right
Credited answer: C
The notes below walk through why it fits the stem and how to eliminate the rest.
Argument or Facts
Argument
Valid or Flawed
Flawed
Question Type
Errors in Reasoning Questions
Stimulus Summary
Research - Extreme insomniac --most--> Drink a lot of coffee
Tom - Drinks a lot of coffee
Conclusion - Tom is likely an extreme insomniac
Answer Anticipation
Note the shift here - we changed “90%” to “most.” While you could have kept it at 90%, the reason that we shifted it here is to note that the relationship is directional - we learn about 90% of extreme insomniacs, but we don’t learn about those who drink a lot of coffee.
While 90% of extreme insomniacs drink a lot of coffee, that could represent a staggeringly small percentage of coffee drinkers. It’s possible that there are only 2,000 extreme insomniacs in the world, with 1,800 of them drinking a lot of coffee. That might represent a very small percentage of the coffee drinkers of the world!
In essence, this argument commits an illegal reversal, flipping around a “most” statement, which can’t be done. Another way of phrasing it is that we learn a lot about one group (extreme insomniacs, almost all of whom drink a lot of coffee), but then it’s applied to another group (those who drink a lot of coffee).
Let’s find an answer reflecting this error in reasoning.
Answer choices
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AIt fails to acknowledge Remaining source text redacted.
Why choice A is not credited
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. -
BIt fails to consider Remaining source text redacted.
Why choice B is not credited
The argument isn’t about causality, but the overlap between groups. As such, other causes of extreme insomnia are out of scope. -
CIt relies on evidence Remaining source text redacted.
Why choice C matches the stem
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. -
DIt draws an inference Remaining source text redacted.
Why choice D is not credited
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. -
EIt presumes without warrant Remaining source text redacted.
Why choice E is not credited
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.
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