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Complete Question Explanation

Here, the salesperson makes an argument comparing a person’s “old vacuum cleaner” to the Super XL vacuum cleaner. In doing so, the salesperson conducted an experiment: he first ran the old vacuum cleaner over a dirty carpet, and then ran the Super XL over the same carpet. Because the Super XL picked up dirt on the carpet left after the old vacuum cleaner was used on it, he concludes that the Super XL is the better vacuum.

The experiment’s structure definitely leaves much to be desired. Without getting bogged down in thinking about all of the reasons why the experiment was flawed, or how it should have been designed, go into the answer choices looking for one that describes how the amount of dirt picked up by the Super XL after the old vacuum cleaner had already gone over the same carpet does not
establish that the Super XL is the better vacuum cleaner.

Answer choice (A): The possibility in this answer choice would be relevant to an argument that concluded something like “the Super XL leaves no dirt behind,” but is not relevant to the comparison between the old vacuum cleaner and the Super XL.

Answer choice (B): There is no indication that the salesperson has made any inference whatsoever about the performance of the Super XL at a specific time in the future. All the stimulus discussed is the comparative performances of the machines at the time of the test.

Answer choice (C): This answer choice is inconsistent with the stimulus, which did not reach an absolute conclusion that the Super XL is the best vacuum cleaner. Instead, the conclusion simply compared the Super XL to the old vacuum cleaner.

Answer choice (D): This is a tricky answer choice. It is true that the author does not discuss the possibility that the old vacuum cleaner removed more dirt than the Super XL. However, that comparison, as to the total amount of dirt removed, is not the basis of the salesperson’s conclusion. Rather, the conclusion is based on the the Super XL removing dirt that the old vacuum cleaner did
not. To know which vacuum cleaner is superior, the experiment needed to test the vacuum cleaners under the same conditions. This answer choice does not describe that flaw, and so is incorrect.

Answer choice (E): This is the correct answer choice. As mentioned above, this answer choice is quite similar to answer choice (D). What makes this answer choice correct is the answer’s focus on the Super XL removing dirt that the old vacuum cleaner did not. If the Super XL and the old vacuum cleaner would have left behind the same amount of dirt when faced with the same carpet in the same state, then the salesperson’s conclusion would be shown false. The salesperson does not deal with this possibility, which is a flaw in the argument.
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Why doesn't D disprove the argument?
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Hi amacmill,

The salesperson is claiming that the Super XL vacuum is superior because it picked up dirt the other vacuum left behind. He's not claiming that it is superior because it picked up more dirt overall.

Answer choice (D) would be correct if the salesperson had claimed that the Super XL picked up a greater amount of dirt than the standard vacuum did.

However, the salesperson instead claimed that the Super XL picked up dirt that the regular vacuum could not. Answer choice (E) attacks the salesperson's argument since it raises the possibility that, if the XL had been used first, it would have performed just like the regular vacuum did. The test was unfair, and answer choice (E) highlights the potential flaw in how the two vacuums were tested.

I hope that helps clarify things! :) Good luck studying!

Athena Dalton
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I'm a little confused by the explanation of this question which states the issue is not the AMOUNT of dirt picked up by the vacuum cleaners.

Both answer choices refer to the AMOUNT of dirt picked up or left behind (which is after the amount left after being picked up).

I chose D because if the amount of dirt the old vacuum picked up was huge and it only left a few specks of dirt, then even a weak vacuum could remove those few specks. So, the XL may not be so much better than the old vacuum that picked up huge amounts of dirt.

Seems to me that both D and E weaken the conclusion.
 Brook Miscoski
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In a nutshell the sales person wants us to believe that the new vacuum is better at picking up dirt. He's ignoring that on the second pass there wasn't as much work to do.

D is wrong because the question isn't about the comparative amounts actually removed, but rather the total ability of each vacuum cleaner to pick up dirt.

E is right because it hits on the fact that on the second pass there wasn't as much work to do.

Another way to think of this is that the sales person is doing a comparison on unequal footing. The only way we can truly compare the two vacuums is to have each of them start from the same conditions. Again, E points out that we don't know what would happen if they started from equal conditions.
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Would this flaw be classified as an error in the use of evidence? If so, would this be considered "some evidence for a position is taken to prove that position is true? I want to be sure I fully understand and continue to identify this type of flaw in the future.
 Jeremy Press
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Hi Leela!

That's certainly one accurate way of understanding the reasoning flaw in this question. There is some evidence of superiority (the Super XL vacuum picked up dirt that the old vacuum did not). But that evidence does not by itself prove the conclusion that the Super XL vacuum is better overall.

But it's interesting that the answer choice does not directly describe the flaw in that kind of formal way. Normally the direct/formal description of that kind of evidence error is something like, "the argument treats evidence showing mere plausibility as if it proves that the conclusion is in fact true."

Instead, the writers of the test chose to do something that is becoming a little more popular on recent tests. Rather than focus on a formal description of the flaw, they ask you to see a possibility that illustrates the presence of the flaw. Answer choice E is a possible fact that, if true, illustrates that the conclusion does not follow certainly from the premise. Keep an eye out for such answers on Flaw questions with Evidence Errors. They'll usually be introduced by some variant of a phrase that indicates the argument has ignored or overlooked a possibility or fact.

I hope this helps!

 James Finch
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Hi Alan,

This is a relatively rare flaw type, where there's no control group for an experiment. So before getting into the answer choices directly, you'd want to Prephrase what an controlled experiment would look like and what it could show as a result. In this case, we only have the Super XL following the old vacuum, but don't have any data on what happens when the Super XL is run first, and then the old vacuum run second. The Super XL could end up leaving behind just as much or even more dirt, which the old vacuum could then pick up.

(D) is attractive if you haven't thought through what a valid experiment would look like: the experiment should show which vacuum would pick up more dirt on a first pass over a dirty carpet, not the relative amount between a first and second pass. And a second pass should, by its very nature, have less dirt available to pick up. So a solid Prephrase is key to getting this and other LR questions that have an attractive wrong answer choice correct.

Hope this helps!

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