Logical reasoning PrepTest 124 · Section 1 · Question 13

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Some doctors: A certain drug helps vertigo. Since the drug was introduced, the average vertigo attack length is lower.

Rebuttal: A recent shortage didn't see a change in vertigo attack length, so the drug doesn't work.

Answer Anticipation:
There are two elements in this stimulus that suggest specific flaws/assumptions that we can analyze to find a necessary assumption.

The first is the type of conclusion here—an anti-causal conclusion. This type of conclusion suggests that a causal connection doesn't exist, though still usually based on a correlation. Here, the argument concludes that a drug doesn't work because a change in the availability of the drug didn't affect a change in the disease it's meant to treat. This type of argument is generally flawed because it doesn't compare the effect to what it would have been with the cause still present. For example, it states that traffic laws don't work because high accident rates are correlated with areas that have them. However, since this argument looks at a more complete data set—everyone who suffers from vertigo, and what it looked like before, during, and after the drug was generally available—this flaw isn't likely to yield a correct answer.

Which leads us to the second hint—the stimulus has a timeline, with, as we just said, information about before, during, and after the drug was generally available. It specifically notes that the time period featured in the premise that the argument crucially relies on is a 3-month period where the drug had a shortage. The argument, therefore, assumes that a 3-month shortage is enough to see the effect of not having the drug present. It's possible that the drug takes more than 3 months to clear from the system, or everyone who needs it has a supply that would last more than 3 months, or that it's some type of implant that lasts for quite some time.

This argument is assuming that a 3-month shortage is enough for the effects of the shortage to be seen in those suffering from the disease, so let's find an answer that establishes this.

Answer Explanation:
This answer addresses the assumption related to the timeline in the argument. If a three-month shortage wouldn't be long enough to show the effect on those suffering from vertigo, then the argument falls apart since the lack of a change in average duration wouldn't show that the drug doesn't work. This answer is necessary for the argument to work, so it's the correct answer.

Key Takeaway:
Whenever an argument brings up a timeline, think about whether it relates to the conclusion. Here, by limiting the collection of evidence to a three-month period, the argument opened itself up to criticism that it didn't consider a long enough time period.

Incorrect. If anything, this answer is necessary for the claim made by "[s]ome doctors." That said, the author of the argument never establishes whether there actually was a decrease in the average donation after the drug was introduced—it's only stated as something that the opposing point "claim[s]" in supporting their point.

Incorrect. Even if a longer time period would have been better evidence, that doesn't mean a three-month period isn't good evidence for the conclusion. While this answer does address the timeline assumption, it doesn't do so in a necessary way.

Incorrect. Since the argument rules out the drug as being responsible for any change in vertigo attack duration, if there is such a change, the argument relies on an alternative cause being responsible. This answer, in ruling out alternatives, cuts against that conclusion.

Incorrect. If there's only a single other relevant factor, that could explain any change in the length of attack instead of the drug, so the argument doesn't assume that there are "various significant factors."