LSAT 151 – Section 3 – Question 13

LSAT 151 - Section 3 - Question 13

September 2018

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Question
QuickView		 Type Tags Answer
Choices		 Curve Question
Difficulty		 Psg/Game/S
Difficulty		 Explanation
PT151 S3 Q13
+LR
Flaw or descriptive weakening +Flaw
Conditional Reasoning +CondR
A
91%
162
B
3%
149
C
4%
153
D
2%
150
E
1%
148
134
141
149
 +Easier 146.292 +SubsectionMedium

A new screening test has been developed for syndrome Q. Research has shown that the test yields a positive for syndrome Q whenever the person tested has that syndrome. So, since Justine shows positive on the test, she must have syndrome Q.

Summarize Argument
The author concludes that Justine has syndrome Q. He supports this by saying that if someone has syndrome Q, they test positive, and Justine tested positive.

Identify and Describe Flaw
This is the cookie-cutter flaw of confusing necessary and sufficient conditions. The author treats “positive” as sufficient for “syndrome Q,” but according to his premises, “positive” is necessary.
In other words, just because Justine tested positive doesn’t necessarily mean that she has syndrome Q. She might just have had a false positive test.

A
It confuses the claim that a subject will test positive when the syndrome is present with the claim that any subject who tests positive has the syndrome.
The author treats testing positive as sufficient for having syndrome Q. But his premises say that it’s necessary: if someone does have syndrome Q, then they will test positive, not the other way around. So Justine might not have syndrome Q, even though she tested positive.
B
It makes a general claim regarding the accuracy of the test for syndrome Q without providing adequate scientific justification for that claim.
The author claims that the test is accurate and positive when someone does have syndrome Q. He doesn’t necessarily need to provide scientific justification for this, but he does say that research has proven it to be true.
C
It fails to adequately distinguish between a person’s not having syndrome Q and that person’s not testing positive for syndrome Q.
Instead, the author fails to distinguish between a person testing positive for syndrome Q and that person having syndrome Q. He doesn’t mistake a person not having syndrome Q with that person not testing positive for it.
D
It confuses a claim about the accuracy of a test for syndrome Q in an arbitrary group of individuals with a similar claim about the accuracy of the test for a single individual.
The author never confuses his claim about the test’s overall accuracy with his claim about Justine’s test. Also, we have no reason to believe that the research used an “arbitrary group” when measuring the test’s accuracy.
E
It confuses the test’s having no reliable results for the presence of syndrome Q with its having no reliable results for the absence of syndrome Q.
If someone does have syndrome Q, then they test positive. So the test does have reliable results for the presence of syndrome Q. We don’t know whether it has reliable results for the absence of syndrome Q, but regardless, the author never confuses the ideas presented in (E).

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LSAT PrepTest 151 Explanations

Section 1 - Reading Comprehension

Section 2 - Logical Reasoning

Section 3 - Logical Reasoning

Section 4 - Logical Reasoning

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