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#36290
Complete Questions Explanation

Flaw in the Reasoning. The correct answer choice is (B)

In this stimulus, the author presents the fact that defense attorneys sometimes try to acquit suspects
by comparing DNA samples from the suspect to DNA samples from the crime scene. But, even
though every person has unique DNA, sometimes the DNA test does not distinguish between
different people. Thus, according to the author, it is a mistake to exonerate a suspect just because the
DNA of the suspect does not match the DNA from the scene.

Most students read the stimulus and think that the reasoning is valid, and so they are surprised when
the question stem asks for the fl aw in the reasoning. When this occurs, you should immediately
glance at the stimulus again to see if you missed an important word or phrase. If you did understand
the stimulus, use the answer choices to get a better sense of what fl aw you might have missed.

The fl aw in the reasoning is discussed in detail in the discussion of the correct answer choice.

Answer choice (A): This choice is wrong, because if the argument assumed that evidence can never
be mistaken, then the argument would not have concluded that DNA evidence can be mistaken.
Additionally, this answer addresses physical evidence, a broader concept than DNA.

Answer choice (B): This is the correct answer choice. This choice can be somewhat confusing
to read, and many students select this answer not so much because they understand exactly what
it means but because they know the other answers are incorrect (and such an approach is perfectly
valid on the LSAT).

In reality, these DNA tests are imperfect because they “sometimes fail to distinguish among samples
taken from distinct individuals.” That is, the test sometimes “incorrectly identifi es DNA samples as
coming from the same person.” So the test sometimes produces false positives. Based on this fact,
the author mistakenly believes the test also produces false negatives; that it cannot reliably rule out
suspects whose DNA doesn’t match crime scene samples.

In other words, this is the fl aw in the reasoning, and the point that the author apparently missed: Just
because the test cannot distinguish every sample of DNA from every other sample, this does not
necessarily mean that it is unable to recognize a clear mismatch.

Answer choice (C): This response is incorrect because the stimulus discusses only DNA evidence
and does not discuss the reliability of “all methods.”

Answer choice (D): There is no indication that the author has relied on data that did not hold up
under non-experimental questions, and this answer choice certainly does not describe the logical fl aw
the author made in moving from the premises to the conclusion.

Answer choice (E): The issue of other evidence aside from DNA evidence is irrelevant because the
author addresses only DNA evidence (and, if anything, the stimulus actually supports the idea that
other evidence is required, not that other evidence should not be admitted).
 hfsports429
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#31936
Hi,

Could someone explain Answer choice B? I'm having a lot of trouble understanding the language of the answer-as in the literal meaning especially in the context of the stimulus.

Thank you!
 David Boyle
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#31971
hfsports429 wrote:Hi,

Could someone explain Answer choice B? I'm having a lot of trouble understanding the language of the answer-as in the literal meaning especially in the context of the stimulus.

Thank you!

Hello hfsports429,

"It confuses a test that incorrectly identifies DNA samples as coming from the same person with a test that incorrectly shows as coming from different persons samples that come from a single person." The first test mentioned apparently refers to, "DNA tests often fail to distinguish among DNA samples taken from distinct individuals". Knowledge of that DNA-test failure would actually help with exoneration, since that failure shows that there might be a false ID, falsely "confirming" that you might be the same as some other, really guilty person. Thus, you might get mistaken for someone else who actually did the crime.
But the stimulus claims that there should be no exoneration, which goes against the logic of what was said above.
Now, by contrast, if "a test that incorrectly shows as coming from different persons samples that come from a single person" were the right way to characterize DNA tests, that would help fight against exoneration, since that test should ID you as a criminal--if you were a criminal--, but it lets you off the hook because it denies your identity and blames someone else.
Hence, answer B is best because it describes the bait-and-switch that the author tries to pull on readers, by confusing the two very different test-failure trends with each other.

Hope this helps,
David
 adlindsey
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#32169
I still don't understand the explanation; it's too abstract. Maybe a scenario/demo or example might help.
 Kristina Moen
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#32200
adlindsey wrote:I still don't understand the explanation; it's too abstract. Maybe a scenario/demo or example might help.
You got it!

One of the premises is that "DNA tests often fail to distinguish among DNA samples taken from distinct individuals." So, you've got John and Adam, and their DNA samples look the same. Imagine if John actually committed the crime and left some DNA (hair or blood sample, you pick!). Adam is a suspect in the crime, so his DNA is tested, and it's a match! What we learn is that just because your DNA matches the crime scene, it doesn't mean that you actually committed the crime. This information would probably lead you to want to exonerate people - some innocent people (like poor Adam!) could be behind bars.

However, the author concludes that it is a mistake to exonerate someone just because their DNA does not match the DNA samples from the crime scene. So let's refer to the above example. John and Adam have DNA samples that look the same. Adam is a suspect in the crime, so his DNA is tested, and it's not a match. Well then, could the DNA still be Adam's? No. Could it be John's? No. Perhaps their DNA has markers in common, which the DNA test picks up. But if the DNA test doesn't pick up those markers, then it's not either of their DNA.

Here's another example. John and Adam have red sweaters. The Sweater Test (stay with me here!) can only identify color, so it can't distinguish between John and Adam's sweaters. A sweater is found at the crime scene. But the sweater is yellow, so the Sweater Test reveals it's not John's. According to the argument in the stimulus, we can't exonerate John. But that's confusing a test that assigns the same sweater to different people (John and Adam) with a test that assigns different sweaters to the same person (John).

Hope this helps!

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