LSAT 151 | Section 3 | Logical Reasoning: Q13

LR Question 13 Explanation

QUESTION TEXT: A new screening test has been developed for syndrome Q …

QUESTION TYPE: Flawed Reasoning

CONCLUSION: Justine has syndrome Q.

REASONING: The test for syndrome Q is always positive when the person has the syndrome. Justine had a positive test.

ANALYSIS: The evidence only tells us that if a person has the syndrome, the test will show positive. So there won’t be any cases where you have the syndrome but the test doesn’t detect it.

That doesn’t mean that if it says positive, you always have it. It doesn’t rule out the possibility of false positives. Justine may have gotten a false positive.

“Everyone who has Q tests positive” (reasoning) isn’t the same thing as “everyone who tests positive has Q” (conclusion).

___________

1. CORRECT. The argument treats a positive result as proof of the syndrome, even though the premise only says people with the syndrome always test positive.
2. We’re supposed to take premises as true. If research has shown that it’s good at detection, then we should presume it is.
3. This would be about confusing not having Q with not testing positive. But the argument is about a person who does test positive.
4. There’s no whole-to-part flaw here.
5. This talks about reliable results for presence versus absence, but the stimulus never says the test has no reliable results. It says the test reliably gives positives when Q is present.