2 Tough LSAT Logical Reasoning Flaw Questions

This article was inspired by a discussion with one of my LSAT students. We were discussing the flawed reasoning in a particular Logical Reasoning question. That question's flaw reminded me of a very similar flaw in another LSAT question.

My student was kind enough to find the other question for me, so I'm placing the two side-by-side in this blog post.

This flaw is much less common on the LSAT than the typical necessary/sufficient or correlation/causation confusion. However, familiarizing yourself with it now will make it easier to recognize if you end up seeing it on your exam.

It's the flaw of confusing a false positive with a false negative.


Example #1:

PrepTest 11 (June 1994), Section 2, Question 15 (p125 in 10 Actual)

The question deals with computer security and whether the system grants access to the wrong people.

Evidence: The computer has never committed a false positive determination. In other words, it never allowed the wrong people to access the computer system.

Conclusion: Therefore, it will probably allow the right people, and only the right people, access to the computer system. In other words, the computer system will probably never commit a false negative determination, either.

This argument is flawed. Based on the evidence, it's possible the computer system could simply be super-sensitive and not grant access to anyone. At least, it's possible that the computer system may fail to grant access to many of the right people.


Example #2:

PrepTest 45, Section 1, Question 24

The question deals with DNA tests, criminal proceedings, and exonerations of suspects.

Evidence: DNA tests often confuse two different people's DNA samples as being from the same person. In other words, a particular DNA sample might not actually match DNA taken from a crime scene, but this test will falsely confirm that the two are a match.

Conclusion: Therefore, we should not trust the DNA test when it says that a suspect's DNA does not match DNA taken from a crime scene.

This argument is flawed. The evidence tells us that the test confuses different DNA samples and wrongly leads to convictions. In other words, we know it makes false positive determinations, making innocent people appear guilty.

However, this doesn't mean the test mistakenly tells us that DNA samples do not match those from a crime scene (which would wrongly prevent convictions). In other words, the evidence does not give us reason to believe the DNA test makes false negative determinations, which would make guilty people appear innocent.

Photo by ynse / CC BY-SA 2.0