LSAT 4 | Section 1 | Logical Reasoning: Q23

LR Question 23 Explanation

QUESTION TEXT: When 100 people who have not used cocaine are tested…

QUESTION TYPE: Flawed Reasoning

CONCLUSION: If a randomly chosen group of people is tested for cocaine use, the vast majority who test positive will be cocaine users.

REASONING: There is a 5% rate of false positives, and only a 1% rate of false negatives.

ANALYSIS: This question can be very counterintuitive. The error rate is lower for people who have actually used cocaine. Doesn’t that mean the test is accurate?

No. It depends on how likely it is that your test subjects have used cocaine. Suppose you tested 100 nuns and monks. Presumably, they haven’t used cocaine. Yet 5 of them will test positive. That is a monstrous error rate.

If this test was used for drug testing at your company, 5% of the employees would get fired, because the test screwed up and labeled them cocaine users.

___________

1. A value judgment would be something like “we should use this test.” The stimulus didn’t make a value judgment.
2. This is like saying: The average man is 5’10”, therefore every man is 5’10”. It’s an error, but a different error.
3. CORRECT. Yes. If you test an addiction clinic, most of your positive results will have actually used cocaine. If you test a maternity ward then most of your positives will (hopefully) be babies who have never used cocaine.
4. It takes this into account but notes that the error rate is lower.
5. The stimulus doesn’t advocate anything: it makes no value judgments.

Recap: The question begins with “When 100 people who have not used cocaine are tested”. It is a Flawed Reasoning question. Learn how to master LSAT Flaw questions on the LSAT Logical Reasoning question types page.