PrepTest 118, Section 1, Question 24
In criminal proceedings, defense attorneys occasionally attempt to(79 more words not shown)
Which one of the following(7 more words not shown)
- AIt assumes without(13 more words not shown)
- BIt confuses a(30 more words not shown)
- CIt generalizes about(29 more words not shown)
- DIt relies on(16 more words not shown)
- EIt fails to(25 more words not shown)
Reading the argument
Whose hair did we find at the crime scene? A mismatch with a random hair wouldn't settle whether the suspect was there. The hair could belong to the victim, an accomplice, or someone who visited earlier.
But suppose we know that a particular blood sample came from the person who committed the crime. A reliable result showing that the sample isn't the suspect's could then be important evidence in the suspect's favor. So the usefulness of a mismatch depends on what the sample tells us and what the test can establish.
The author offers a particular reason to distrust mismatches: DNA tests sometimes fail to distinguish DNA from different people. Let's accept that limitation. But how does it establish that a mismatch cannot provide useful evidence in the circumstances we just considered? We need a reason to dismiss that possibility, not just the observation that tests can sometimes go wrong.
What we’re looking for
For the flaw question, my initial concern is that the argument dismisses evidence that could be useful in some circumstances. We need to be more exact about the reason it gives. Keep the stated test error separate from the test result the author wants us to reject.
Working the answers
- A
The author does not assume that physical evidence is never mistaken. The whole argument begins with a limitation of DNA testing and uses that limitation to question a use of the evidence. A claim of perfect reliability would not describe this reasoning.
- BCorrect
The premise describes a false match: samples actually come from two different people, but the test fails to tell them apart. Imagine one sample from Ana and another from Ben, with the test reporting a match.
The conclusion concerns a different result: the suspect's sample does not match the crime-scene sample. To challenge that result on the same kind of reliability ground, we'd want evidence that a test can falsely report a difference between samples from the same person. For example, two samples from Ana might be reported as not matching.
Those are two different errors. A test might mistakenly group different people's samples together while accurately reporting a difference whenever it finds one. Being told that false matches occur therefore doesn't establish that nonmatches are unreliable. The author treats the first problem as evidence of the second without supplying the connection.
- C
The argument stays with DNA testing. It doesn't use DNA's limitations to reject fingerprints, eyewitness identifications, or every other method of identifying people involved in crimes. The problem is the switch between two kinds of DNA test error, not a generalization to all methods.
- D
We aren't told that the evidence comes only from laboratory experiments or that conditions outside the laboratory differ. The described limitation could have been observed in actual investigations. D introduces a distinction that the argument doesn't make.
- E
The argument doesn't need to establish that physical evidence is the only admissible evidence. Other information could matter too. The issue here is whether the stated problem with DNA tests supports rejecting a particular use of DNA results; rules about admitting other kinds of evidence don't identify that mistake.
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