Try a Sample Logical Reasoning Question: Parallel Flaw

Test 73, Section 4, Question 18

So far this summer there has been no rain in the valley. But usually a few inches of rain fall there each summer. Since only one week of summer is left, it will probably rain in the valley within the next week.


The flawed pattern of reasoning in the argument above is most similar to that in which one of the following arguments?


A

Aisha has finished proofreading all but the last two pages of an issue of the journal Periodos and has encountered no errors. However, there are sometimes a few errors in an issue of the journal Periodos. So there may be errors in the pages that Aisha has not yet checked.

B

There are generally few errors in an issue of the journal Periodos. Aisha has finished proofreading all but the last two pages of an issue of this journal but has encountered no errors. Hence, there are probably no errors in the pages that Aisha has not yet checked in this issue of the journal.

C

On average, there are a few errors in an issue of the journal Periodos. Aisha has finished proofreading all but the last two pages of an issue of this journal but has encountered no errors. So there are probably errors in the pages she has not yet checked in this issue of the journal.

D

Aisha has proofread several issues of the journal Periodos and has encountered no errors. But there are seldom any errors in an issue of this journal. So there will probably be no errors in the next issue of the journal Periodos that she proofreads.

E

There usually are errors in each issue of the journal Periodos. Since Aisha has finished proofreading the latest issue of this journal and has detected no errors, Aisha has probably made a mistake in her proofreading.

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View Explanation

Superbad logic here. This argument is like saying “on average, if you flip a coin 10 times it will come up heads 5 of those times. I just flipped five tails in a row, so it’s going to come up heads the next five times.” It’s dumb because it misunderstands that averages are made up of data points that have variance.

Having spotted the flaw and having made an objection, it shouldn’t be too difficult to find another example of it.

A. I thought this was gonna do it but it saved itself at the last minute. If it said “these last two pages will probably have errors,” this would be the answer. But it went with “may” have errors, which I can get down with. That’s logically valid. Aisha won’t know until she checks those pages. It’s not a flaw to say that there might be errors in them.

B. This is flawed because the last two pages still might have errors. But we needed “will probably have errors,” not “probably won’t have errors” in order to match the given argument.

C. Perfect. Usually, there are “few errors” in an issue of Periodos (there is usually a few inches of rain each summer in the valley). Aisha has read all but the last two pages of an issue of Periodos, but hasn’t come across any errors (there’s only one week of the summer left, and there still hasn’t been any rain). So, there are probably errors in the last few pages (it will probably rain in the valley next week).

D. Nah. This isn’t even flawed. If there are rarely errors, then there probably aren’t errors in the next one.

E. This seems logically valid. And it’s nothing like the given argument anyway. If there usually are errors in the journal, and Aisha didn’t detect any, then it’s likely that she made a mistake in her proofreading.

The answer is C.

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