Logical reasoning PrepTest 109 · Section 1 · Question 17

Incorrect. The fact that an argument could be stronger doesn't weaken it. If this answer stated that 100 test drops are the minimum required to reach valid data, then it'd weaken the argument, but just bringing up a way the argument could be stronger doesn't weaken the argument. At most, it points out a weakness already present in the argument, which is subtly different than actively weakening the argument.

Incorrect. Be careful—there's no indication if the Expert's testimony is helping the prosecution or defense. In fact, in testifying that the defendant's blood doesn't match the expectations, it would appear that the Expert is exonerating the defendant!

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
EW: We ran a blood drop test (is that a thing?) 10 times in controlled circumstances. The results all resulted in a stain ~4.65cm, less than the expected 9.5cm. Therefore, the defendant's blood leaves a stain smaller than 9.5cm.

Answer Anticipation:
The argument has two features that suggest specific flaws that we could focus on to weaken the argument.

First, it relies on the testimony of an expert, so we should consider an appeal to an inappropriate authority flaw. However, there's no indication that this expert doesn't have sufficient expertise and experience to speak about blood drop test results. If an answer establishes that this expert witness doesn't know anything about blood drops, then that would weaken the argument, but that's unlikely since there's no hint of such an issue in the stimulus.

Second, it relies on a study/test, suggesting a potential sampling flaw, and a correlation/causation flaw. The conclusion isn't causal, so we can skip over that analysis. However, there are two issues with the sample. First, it was done in controlled circumstances. We don't know that this is representative of the circumstances that were present at the time of the crime. It's possible that, say, it was raining and fabric being wet might affect the blood drop stain area. Second, it was done ten times—that might not be a sufficiently large number of trials. Maybe the next 20 tests would all show a stain of 20cm, suggesting that 9.5cm is more than possible

Since there's information in the stimulus that establishes a sampling flaw is possible, let's focus on answers that call the sample into question—either because the actual conditions during the crime weren't controlled, or because 10 is just too low a sample size to draw the conclusion.

Answer Explanation:
This answer plays off of the small sample. If an 11th drop left a much larger stain, then it could be the case that those first 10 trials were outliers. More tests would have to be done to know that the defendant's blood couldn't have left the stain, weakening the conclusion here.

Key Takeaway:
In general, a sample size of ten or fewer will lead to an answer about that sample size being too small; a sample size under 100 will often lead to that answer; and a sample of 100+ will be assumed to be large enough. There are exceptions to this, but don't anticipate a too—small sample answer without a very small sample!

Incorrect. This answer suggests that blood from different people leaves different stains, which, if anything, lines up with the argument by supporting the possibility that someone else's blood is in the stain.

Incorrect. Without providing a reason to believe that this expert witness isn't as much of an authority as claimed, this answer is too generic to weaken the argument.