Logical reasoning PrepTest 151 · Section 3 · Question 13

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
If someone has Syndrome Q, they'll test positive on the new test. Justine tested positive, so she must have Syndrome Q.

Answer Anticipation:
Straight up illegal reversal of the conditional. If someone has the disease, they'll test positive; if they don't test positive, they don't have the disease. Justine tested positive, which isn't a sufficient condition in the conditional, so no valid conclusion can be drawn.

Answer Explanation:
This answer, in a slightly convoluted way, states that having the syndrome is mistakenly treated as necessary for a positive result instead of sufficient for it. It's the correct answer.

Key Takeaway:
Conditional logic errors can frequently feel valid or repetitive, since the ideas mentioned overlap so strongly. Whenever there's a conditional statement in an Errors in Reasoning question that is applied to a situation, make sure that situation establishes a sufficient condition as a premise and concludes a necessary condition—if it doesn't, there's an illegal reversal.

Incorrect. This answer doesn't necessarily reflect a flaw. If the LSAT states a test is accurate, then it should be treated as accurate. While that doesn't happen here, the flaw isn't that it's trusting science.

Incorrect. Justine both tested positive and is concluded to have Syndrome Q, so this answer describes a situation that isn't in the stimulus.

Incorrect. This argument does make a general statement about accuracy before applying it to one individual, but it doesn't confuse those claims. This answer would be correct in an argument that said most test results taken yesterday were correct, so Justine, who was tested yesterday, had correct test results.

Incorrect. The argument doesn't discuss reliability at all, so this answer is out of scope.