PT45.S1.Q24 - in criminal proceedings

extramedium

Still trying to tease out the stimulus on this one. I don't understand what the two different DNA tests are or how they are confused or what the assumption is.

Premise: DNA tests can't distinguish among samples from different people. Samples A and B are both different, but test says they are both A.

Conclusion: You can't exonerate someone because their DNA did not match the DNA at the scene of the crime. Sample A did not match Sample X (crime scene), thus, one would expect A and B to be exonerated since they were both identified as A. Or that B would be exonerated anyways because the tests says his DNA is also A. But the author is saying, no, you cannot exonerate sample B because B was misidentified as A, so B could still be the assailant.

I was able to eliminate all the other answers. This is just one where I can't understand why AC B is correct. Let me know if I misunderstood the premises here.
https://7sage.com/lsat_explanations/lsat-45-section-1-question-24/

Sarah889

Hi there!

This is definitely a tricky question. The wording of the correct AC especially. Let's start by identifying the structure of the stimulus.

Context: A method employed by defense attorneys to establish that a suspect was not at the scene during the crime is by comparing this suspect's DNA to the DNA taken from the scene of the crime.
P1: It is true that every person's DNA is unique.
P2: It is also true that DNA tests often fail to distinguish among DNA samples taken from distinct individuals.
C: Therefore, we should never exonerate a suspect if the basis for doing so is only because that person's DNA did not match the DNA samples taken from the scene.

The author is taking issue with using DNA testing in order to exonerate suspects because DNA testing does not reveal the distinct individual from which the DNA came from.

So, let's illustrate this by an example. Consider your blood type. Every person has unique DNA, but there are some characteristics about our DNA that are shared. Many people have a certain blood type, while each of those people have unique DNA. What the author is saying is that, the DNA tests do not distinguish between distinct individuals; however, what B is pointing out is that this doesn't mean these tests don't distinguish between groups that share a certain characteristic.

For example, if the blood type that was found at the scene was A negative and your suspect is B positive, the DNA comparison would be an effective way to exonerate the suspect, regardless of the author's objection.

Now let's look at B... In English, B is essentially saying that, just because the DNA tests fail to distinguish which exact person the DNA came from, doesn't mean the DNA tests also fail in giving us more information about the individual, which may allow us to exonerate him/her.

Understanding the wording of this AC is half the battle in my opinion... Here is how I understand it: It confuses a DNA test that fails to reveal the exact person with a DNA test that incorrectly identifies certain characteristics of the DNA, which make it so that samples coming from the same person are shown as coming from different people.

In order for DNA tests to be as useless as the author asserts, the DNA tests have to either 1) give us no identifying information whatsoever or 2) render incorrect readings. B utilizes option 2. The flaw is that the author takes the fact that DNA testing can't draw us a map to the perpetrator as assuming that DNA testing render incorrect results, which, as illustrated with the blood type example, isn't necessarily true.

Hope this helps.

BinghamtonDave

I found the stimulus rather nebulous and difficult to get a firm grasp on when I first looked at the question, but during a thorough breakdown, think I have a decent grasp on it. First, we should start with some running trends throughout flaw questions: with flaw questions, we are looking for a description of why the conclusion given is not supported by the premises provided.

Inevitably, there will be a gap, a lacuna if you will, between the premises and the conclusion. This case is no different.

The core of this problem:
We are given evidence in the second sentence that DNA testing often fails to distinguish between distinct people. Essentially, if a hair of mine was left at the scene of a crime, this failure would indicate that that hair matched any number of people: my town's mayor, my primary care doctor, my father in law etc. By extension, this failure places any number of people at the scene of the crime. Think about what this failure does: my hair implicates someone else.

Now, what direction would the conclusion have to go in order to evade a potential flaw, given that premise? How about: therefore, a match on a DNA sample should be taken with caution. This construction would be commensurate with the premise supplied.

Instead, what do they give us on the basis of the test matching my hair to any number of people and by extension placing them at the scene of the crime?
They give us a conclusion totally unsupported by the premise provided. They give us a judgement on the idea of exonerating people whose DNA does not match the sample from the crime.

But, we don't know anything at all about people who were not matched we only know something about people were by some failure were matched.

A digression: What makes this question difficult in my estimation is that it plays to some deeply held biases we all might hold. So for instance, in regular every day conversation, if someone tells me: the DNA test over here places 9 different people at the scene of the crime on the basis of 1 sample. I would say: HOLD UP! And demand a moratorium on ALL DNA samples: this issue is just too serious, innocent people can go to jail! I think it would be perfectly reasonable in every day parlance and morality to say the whole DNA well might be poisoned, given the failure of the tests outlined in the premise.

But, for the purposes of this exam, we are evaluating arguments in a very technical way. The premise provided simply does not support the conclusion given, they are talking apples to bowling balls.

I hope this helps
David

PS: a word on (B) forthcoming