In criminal proceedings, defense attorneys occasionally attempt to est

Question

In criminal proceedings, defense attorneys occasionally attempt to establish that a suspect was not present at the commission of a crime by comparing the suspect’s DNA to the DNA of blood or hair samples taken from the scene of the crime. Although every person’s DNA is unique, DNA tests often fail to distinguish among DNA samples taken from distinct individuals. Hence, it is a mistake to exonerate a suspect simply because that person’s DNA did not match the DNA samples taken from the scene of the crime.

Which one of the following is an error in the reasoning above?

(A) It assumes without warrant that the use of physical evidence in identifying suspects is never mistaken.

(B) 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.

(C) It generalizes about the reliability of all methods used to identify those involved in the commission of a crime on the basis of results that pertain to only a few such methods.

(D) It relies on experimental data derived from DNA testing that have not been shown to hold under nonexperimental conditions.

(E) It fails to demonstrate that physical evidence taken from the scene of a crime is the only sort of evidence that should be admitted in criminal court proceedings.

Bunuel · Accepted Answer

EXPLANATION FROM Fox LSAT

This argument says, “Because DNA tests have a lot of false positives (they might fail to distinguish between two different people, thus showing a match where there actually wasn’t one), therefore they shouldn’t be used to establish the absence of a suspect from a crime scene.” But that doesn’t make sense! It’s true that false positives are a problem if we’re trying to use them to convict someone. If a DNA test can’t tell the difference between me and Mr. Murderer, then it doesn’t seem fair to convict me on the basis of a DNA test. Maybe Mr. Murderer did it! But if the test can’t tell the difference between me and Mr. Murderer, couldn’t it nonetheless be used to prove that  neither  of us was there?

False positives, in other words, aren’t a problem if you’re only using the test to exonerate. False  negatives  would be a problem if we were trying to use the test to exonerate.

Example: Suppose a witness knows that Mr. Murderer and I were both playing ping-pong the entire time the crime occurred. The witness doesn’t know who won the ping-pong match. In fact, the witness doesn’t even know me from Mr. Murderer. But the witness knows neither of us could have done it, because the witness knows we were both playing ping-pong the entire time. Couldn’t that witness be used to exonerate me and Mr. Murderer? Similarly, the DNA test could also be used to exonerate us both, even if it couldn’t tell the difference between the two of us. However, it couldn’t be used to convict either one of us.

We’re asked to identify the flaw, and I think we’ve already nailed it. The argument has tried to say that false positives are a problem for the defense, when actually they’re only a problem for the prosecution.

A) The argument simply doesn’t do this.

B) I think this might mean, “Confuses a test that has false positives with a test that has false negatives.” This is probably the answer.

C) No, the argument never “generalizes about the reliability of all methods.”

D) No, the argument doesn’t “rely on experimental data.”

E) No, the argument doesn’t claim that physical evidence is the only admissible evidence.

The answer is B.

Without even bothering to think too closely about exactly what B means, it must be our answer. The argument definitely didn’t even do what A, C, D, and E are claiming the argument did, so none of them can possibly be the answer. If that’s the case, then B must be the answer even if it seems like gibberish.

winterschool · Answer

In criminal proceedings, defense attorneys occasionally attempt to establish that a suspect was not present at the commission of a crime by comparing the suspect’s DNA to the DNA of blood or hair samples taken from the scene of the crime. Although every person’s DNA is unique, DNA tests often fail to distinguish among DNA samples taken from distinct individuals. Hence, it is a mistake to exonerate a suspect simply because that person’s DNA did not match the DNA samples taken from the scene of the crime.

Which one of the following is an error in the reasoning above?

(A) It assumes without warrant that the use of physical evidence in identifying suspects is never mistaken.  Incorrect

not true

(B)  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.   Correct

in last two line of argument author confuses abut DNA test, this choice represents that error

(C) It generalizes about the reliability of all methods used to identify those involved in the commission of a crime on the basis of results that pertain to only a few such methods.  Incorrect

argument is not talking about any other methods

(D) It relies on experimental data derived from DNA testing that have not been shown to hold under nonexperimental conditions.  Incorrect

not true

(E) It fails to demonstrate that physical evidence taken from the scene of a crime is the only sort of evidence that should be admitted in criminal court proceedings.  Incorrect

not true

sivatx2 · Answer

"Although every person’s DNA is unique, DNA tests often fail to distinguish among DNA samples taken from distinct individuals." This is a flawed premise. How can a test result from same individual show distinction between the DNA samples that was collected? DNA will be always same for an individual and won't vary . Hence, this premise is incorrect.

(A) It assumes without warrant that the use of physical evidence in identifying suspects is never mistaken. 
 Physical evidence is always correct - this is not stated in the argument. Eliminate

(B) 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. 
 This exactly reflects the analysis that the premise is flawed. Hence, B is the correct answer choice

(C) It generalizes about the reliability of all methods used to identify those involved in the commission of a crime on the basis of results that pertain to only a few such methods. 
 There was no generalization made in the argument. This is false statement and hence eliminate.

(D) It relies on experimental data derived from DNA testing that have not been shown to hold under nonexperimental conditions. 
 Experimental vs nonexperimental conditions is not discussed and not relevant to the argument. Eliminate

(E) It fails to demonstrate that physical evidence taken from the scene of a crime is the only sort of evidence that should be admitted in criminal court proceedings .
 Physical evidence is never considered as the only sort of evidence, and is irrelevant to the argument. Eliminate

In criminal proceedings, defense attorneys occasionally attempt to est

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In criminal proceedings, defense attorneys occasionally attempt to est

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