Logical reasoning PrepTest 157 · Section 3 · Question 25

Question prompt

To test the claim Remaining source text redacted.
Why the credited answer is right

Credited answer: A

The notes below walk through why it fits the stem and how to eliminate the rest.

Argument or Facts

Argument

Valid or Flawed

Flawed

Strategy Overview

Argument or facts? Always argument, so identify premises and conclusions of argument Anticipate why the premises are not enough to show that the conclusion is true
Causal argument? Look for answer choice that:
Shows possible alternate cause Shows cause without effect Shows effect without cause Shows the cause and effect are reversed
If not causal argument, anticipate ways to worsen problem with argument
Use anticipations to select the answer choice that, if true, would weaken argument

Answer Anticipation

Our ultimate goal is to find the answer choice that weakens the argument. However, it's much easier to do that if we first break down the argument and identify its flaws. Then, weakening the argument is only a matter of finding the answer choice that exploits that flaw. Causation flaws are among the more common on Weaken (and Strengthen!) questions, and this is technically a causal argument, albeit in the direction of arguing AGAINST a causal relationship. The author argues that vitamin C probably doesn't cause a reduction in acne. To weaken the scientist's anti-causal argument, we could strengthen the cause-and-effect relationship. So, reverse the usual causal anticipations for a Weaken question, and now we're looking for an answer choice that presents something that would strengthen this causal relationship between vitamin C and acne reduction:
Same cause/same effect No cause/no effect Ruling out an alternate cause Ruling out that cause and effect are reversed
Of these, same cause/same effect and no cause/no effect are the most likely correct answers. (The correct answer will typically rule out an alternate cause or show that the cause and effect isn't reversed when the cause-and-effect relationship is supported by a correlation between the supposed cause and effect. This argument establishes no such correlation.) So, the correct answer might present evidence that vitamin C reduces acne in other similar situations (same cause/same effect). Instead, the correct answer could show that, once people stop consuming vitamin C, their acne appears or gets worse (no cause/no effect). There are other ways to conceptualize this flaw. Perhaps you noticed that the premise says that "no difference in the severity of acne was found between the two groups." This compares the current amount of acne displayed by subjects in the two groups. That premise doesn't support the conclusion's contention that "vitamin C has no real benefit in reducing the severity of acne." To measure whether vitamin C reduces acne, we should track if the subjects' acne decreased, stayed the same, or increased throughout the study. A more compelling premise would show that "no difference in acne reduction was found between the two groups." This flaw is a variation of a "concept switch" or "equivocation." The correct answer might exploit this difference, perhaps by showing that the vitamin-C group's acne decreased more than the placebo group's. That said, many things could feasibly weaken this argument. So, we should keep an open mind as we assess the answer choices. We'll start by asking whether the answer choice makes it more likely that vitamin C reduces acne.

Answer choices

  1. A
    The subjects who were Remaining source text redacted.
    Why choice A matches the stem

    This definitely makes it more likely that vitamin C reduces acne. As we anticipated, this shows "same cause/same effect" by presenting evidence that vitamin C reduces acne. If the vitamin-C subjects had a history of "more severe acne" than the placebo subjects, the vitamin-C subjects probably began the study with worse acne. If the vitamin-C subjects ended the study with the same acne as the placebo group, then we have evidence that vitamin C led to more significant reductions in acne than the placebo. (This answer choice also exploits the "concept switch" or "equivocation" we identified.) This suggests that vitamin C can reduce the severity of acne, weakening this argument.

    Because this answer choice weakens the argument in a way that is consistent with our anticipation, we can select it and give the remaining options an, at most, cursory reading.

  2. B
    None of the subjects Remaining source text redacted.
    Why choice B is not credited

    This doesn't show that vitamin C reduces acne. This might weaken that claim if it said that the subjects who were given the placebo took additional doses of vitamin C. In that case, they could have wound up with the same acne as the vitamin-C group because they were also taking vitamin C. (This would be an example of "same cause/same effect," thus strengthening the claim that vitamin C reduces acne.)

    But, as written, this answer choice says that the vitamin-C group wasn't taking even more vitamin C. Nonetheless, they were given vitamin C during the study and still wound up with the same acne as the placebo group. This information is consistent with the conclusion that vitamin C doesn't reduce acne.

  3. C
    During the study, the Remaining source text redacted.
    Why choice C is not credited

    This is irrelevant to the argument, so it doesn't show that vitamin C reduces acne. Even if the subjects' acne was lower than the national average, we could still use the subjects to track whether vitamin C reduces acne. After all, the study didn't support a conclusion about an absolute amount. The argument didn't conclude that vitamin C guarantees one will have little or no acne. (In that case, using subjects with little or no acne would produce misleading results.) The argument instead concluded that vitamin C probably won't reduce acne. We can measure that, even with people with less acne than most.

  4. D
    Some of the subjects Remaining source text redacted.
    Why choice D is not credited

    This answer choice presents tempting evidence that vitamin C can reduce the severity of acne. After all, if subjects given the placebo consumed foods naturally rich in vitamin C, and if those subjects wound up with the same acne as those who were given vitamin C pills, it seems like the vitamin C may have reduced acne in both groups.

    Still, this answer choice has a major red flag: the weak, wiggly word "some." We don't like that language in Weaken questions. Such weak claims rarely provide enough information to meaningfully change the argument. Therefore, these answer choices rarely weaken the argument in question.

    Even if "some" of the subjects from the placebo group ate foods rich in vitamin C, those few subjects could have been among those who knew which pill they were getting. The author completely ignores those subjects when drawing the conclusion. So, this answer choice may not change the argument at all.

    Also, this answer choice doesn't tell us about the diets of other subjects. Perhaps all the subjects — irrespective of their group — ate foods rich in vitamin C. In that case, the group given vitamin C pills would still have consumed more vitamin C. That would show the group that consumed more vitamin C still wound up with the same acne as the group that consumed less. Those results would be consistent with the conclusion that vitamin C doesn't reduce acne.

  5. E
    Some of the subjects Remaining source text redacted.
    Why choice E is not credited

    This is irrelevant. The author completely ignores those subjects who knew they were in the placebo group when drawing the conclusion. The author only relies on the subjects who didn't know which group they were in. So, this answer choice does not change the argument at all.

Question analytics

Based on historical answer selection rates for this question.

Answer choice distribution

  1. A Credited 36%
  2. B 10%
  3. C 8%
  4. D 33%
  5. E 12%

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