Logical Reasoning

PT118 · S1 · Q24 In criminal proceedings, defense attorneys

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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.

Conclusion (Hence)

It's a mistake to exonerate a suspect simply because that person's DNA didn't match the DNA samples takes from the scene of the crime.

Evidence

DNA tests often fail to distinguish among DNA samples taken from distinct individuals.

Evaluate

The author's conclusion is saying, 'We shouldn't just let a suspect go and presume them innocent (exonerate them) simply because the DNA sample we have of them doesn't match the DNA sample taken from the scene of the crime.'

Let's think about that. Why shouldn't the cops let me go, if my DNA doesn't match the DNA taken from the scene of the crime?

The author thinks that even though my sample didn't match the crime scene sample, I might still be the killer. So she apparently thinks it's possible that the sample of blood they got from me at the crime scene doesn't match the sample of blood they got from me in custody. The DNA test yielded a false negative. It said, , even though they are both blood samples from the same person, me.

Meanwhile, the evidence is talking about false positives.

It's saying that DNA tests would often say, "Yup, Patrick's DNA matches the crime scene DNA", even though I wasn't the killer. Let's say Eddie is the killer but he and I have similar enough DNA, that a DNA test fails to distinguish his DNA (from the crime scene sample) from my DNA.

Goal

Based on their evidence, the author's conclusion should be saying,

Instead it's saying that That would require evidence about false negatives.

Let's look for an answer that's complaining that the evidence is dealing with false positives, while the conclusion is dealing with false negatives.

24.

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

  1. It assumes without warrant that

    Too Strong: never

    The author doesn't commit to any extreme assumption that using physical evidence to identify suspects is never wrong.

    5% picked this

  2. Correct

    It confuses a test that

    Why this is right

    This gets at our False Positive vs False Negative distinction. Whenever we see answers structured like confuses X with Y, if it's correct it will mean that X refers to what we were talking about in the evidence while Y refers to how the author is thinking in the conclusion. Can we match up the first half of this answer with the evidence and the second half with the conclusion? Yes. The first half is talking about a DNA test thinking "sample 1 and sample 2 are from the same person", even though that's incorrect; they're really from two distinct individuals. This is a false positive. A false positive comes when the DNA test thinks I'm the killer, but I'm not. The test is comparing my sample to the sample of the killer's DNA (we are two distinct individuals), but since DNA tests often fail to distinguish among DNA samples taken from distinct individuals, it thinks that the killer's DNA matches my DNA. This is what the evidence is about. A false negative happens when I am the killer, but the DNA test says I'm not. The DNA test is comparing the blood sample they took from me in custody to the blood sample they retrieved from the crime scene (both of them are my blood). And the DNA test is saying, "Nope they don't match. Patrick ain't the killer. Go ahead and exonerate him". That's what the second half of this answer choice is talking about, and that's what the conclusion is talking about.

    Skill tested: Flaw · how this choice captures the argument's function is the move to repeat next time.

    73% picked this

  3. It generalizes about the reliability

    Bad Conclusion Method

    Since this answer is structured, "generalizes about X on the basis of Y", we would expect X to match the conclusion and Y to match the evidence. Was the conclusion about "all methods used to identify those involved in the commission of a crime"? Heavens, no. The last sentence isn't even remotely a claim about all methods used to identify who was involved in a crime.

    13% picked this

  4. It relies on experimental data

    Doesn't Rely on Experimental Data

    There's no experimental data that the author is relying on. The author's sole premise is that "DNA tests often fail to distinguish among samples taken from different people". That's not experimental data.

    1% picked this

  5. It fails to demonstrate that

    Too Strong: only

    Fails to establish / fails to demonstrate / takes for granted / presumes / presupposes are all synonyms for "The author assumed". Did this argument need to assume that physical evidence taken from the crime scene is the only type of evidence that should be admitted? Definitely not.

    8% picked this

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