Logical reasoning PrepTest 118 · Section 1 · Question 23

Incorrect. The argument assumes that using the physical evidence is sometimes mistaken, as someone might be guilty despite the DNA test showing they're not a match for DNA at the scene of the crime, so this answer choice doesn't apply.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
DNA tests sometimes show a match between two DNA samples taken from different people, so it's wrong to clear a suspect because their DNA doesn't match DNA from the crime scene.

Strategy Overview:
This answer reflects that the argument jumps from a premise about DNA tests giving false positives (saying that two samples match when they don't) and a conclusion about DNA tests giving false negatives (a sample that matches but a test that says it doesn't), so it's the correct answer.

Answer Anticipation:
This argument deals in some ideas that aren't the most common on the LSAT, but they do come up with some frequency. The premise here brings up the possibility of a DNA test giving a false positive—saying two things match when they don't actually. Here, that's brought up with the failure of the DNA test to distinguish between two samples from different people. When the test fails in this way, it says they match when they don't—a false positive (match).

The conclusion, however, isn't about a DNA match—it's about a situation where the DNA did not match that of the crime scene. The conclusion here is dealing with a different concept—a false negative. This is when a test falsely shows that something doesn't match even though it does—that would give a reason to not clear a suspect just because their DNA doesn't match the scene. It could be a false negative.

The premises, however, only establish that false positives are possible with DNA tests, not false negatives. So the argument jumps between the two concepts, and the correct answer should highlight that.

Answer Explanation:
This answer reflects that the argument jumps from a premise about DNA tests giving false positives (saying that two samples match when they don't) and a conclusion about DNA tests giving false negatives (a sample that matches but a test that says it doesn't), so it's the correct answer.
Key Takeaway:
It's not the most common issue on the LSAT, but the difference between a false positive and a false negative comes up enough that it's worth understanding. A false positive is when something is said to match when it doesn't—here, the DNA samples from distinct individuals that the test didn't distinguish. A false negative is when something that does match is said to not match—which would be the reason to not exonerate someone who doesn't match DNA at the scene of a crime. When an argument jumps between them, it's committing a flaw.

Incorrect. The conclusion is just about DNA tests, not all methods used to identify criminals.

Incorrect. There's no indication that the information in the premises isn't from actual criminal cases instead of from a lab.

Incorrect. If anything, in concluding that DNA evidence is not always conclusive, the argument is implying that other evidence should be admitted. While that evidence may also be physical, that just highlights that this answer choice is out of scope.