In criminal proceedings, defense attorneys occasionally attempt to establish that a suspect was not present at the co...

Victoria on June 1, 2019

Hello @Meckena-Hultin,

This is definitely a tricky one! It is worded quite confusingly.

Let's start by going through the passage line by line.

In criminal proceedings, defence attorneys sometimes try to prove that a suspect was not present when a crime was committed by comparing the DNA of the suspect to DNA found at the crime scene.

Every person's DNA is unique. However, DNA tests often fail to distinguish among DNA samples taken from distinct individuals. This means that a DNA test may fail to distinguish whether a DNA sample was taken from Person A or Person B. In other words, a DNA test may conclude that the samples taken from Person A and Person B were taken from the same person, thereby producing a false positive result.

The passage concludes that it is a mistake to eliminate a suspect from consideration on the sole basis that their DNA did not match the DNA samples taken from the crime scene.

Overall, the passage is arguing that we should not eliminate suspects from consideration in a criminal investigation if their DNA does not match the DNA samples found at a crime scene. The evidence used to support this conclusion is that it is common for DNA tests to produce a false positive, i.e. fail to distinguish between the DNA of distinct individuals.

The question stem asks us to find the error in reasoning made in the passage above.

Answer choice A says that the error is that the author assumes, without good reason, that using physical evidence as opposed to DNA evidence is better because it is never mistaken. This is incorrect because the author does not mention physical evidence, nor do they suggest that it is superior to DNA evidence. They are arguing that a suspect should not be eliminated from consideration simply because their DNA does not match the DNA samples taken from the crime scene.

Answer choice C says that the author makes a generalization about how reliable the methods used to determine whether an individual has committed a crime are based on results that only relate to a subset of these methods. This is incorrect because the author is arguing that shortcomings of DNA testing mean that suspects should not be eliminated from consideration based on the fact that their DNA does not match the samples taken from the crime scene. In this way, the author is making a statement about the reliability of DNA testing based on common results from DNA testing.

Answer choice D says that the author relies on experimental data from DNA testing which is not the same under non experimental conditions. This is incorrect as the author does not distinguish between experimental and non experimental data. They simply state that DNA tests in general often fail to distinguish among DNA samples taken from distinct individuals.

Answer choice E says that the author fails to demonstrate that physical evidence is the only type of evidence that should be allowed in criminal proceedings. This is incorrect as the author does not ever mention physical evidence. This means that they are not trying to prove that physical evidence is the only type of evidence that should be allowed in criminal proceedings and, therefore, do not have to demonstrate this claim.

Answer choice B says that the author confuses a test that fails to identify DNA samples as coming from the same person with a test that incorrectly identifies samples that come from the same person as samples coming from different people. In the passage, the author says that DNA tests often fail to distinguish between distinct individuals, producing a false positive. However, in drawing their conclusion, the author also suggests that DNA tests can produce a false negative, i.e. identifying samples taken from the same person as being taken from different people. This is the flaw in reasoning as the author concludes that we should not eliminate suspects from consideration in a criminal investigation because a DNA test may produce a false negative, whereas the evidence used to support this conclusion claims that DNA tests often produce false positives.

Hope this was helpful! Please let us know if you have any further questions.