PrepTest 141, Section 4, Question 18
So far this summer there has been no(34 more words not shown)
The flawed pattern of reasoning(16 more words not shown)
- AAisha has finished(48 more words not shown)
- BThere are generally(52 more words not shown)
- COn average, there(52 more words not shown)
- DAisha has proofread(41 more words not shown)
- EThere usually are(34 more words not shown)
Reading the argument
What if this is an unusually dry summer?
A few inches of rain usually fall during a whole summer. That doesn't tell us that the last week will probably supply the missing rain after all the earlier weeks have passed without it. We no longer have an entire summer ahead of us; we have one week.
Think about a fair six-sided die. Across 100 rolls, you'd be very likely to get a five at least once. But if the first 99 rolls contain no five, the final roll still has only a one-in-six chance of being a five. It doesn't have to catch up with what we expected from the whole set.
The weather isn't a die, and the passage doesn't tell us that separate weeks of weather are independent. The comparison just helps expose the missing support: a pattern across whole summers doesn't by itself establish a likely event in this particular remaining week.
What we’re looking for
For the parallel flaw, look for an expected event across a whole span, no event through most of that span, and then a prediction that the event probably occurs in the small remainder.
Working the answers
- A
The proofreading setup is close: no errors through nearly all the issue, just as no rain has fallen through nearly all the summer. But the conclusion says there may be errors, not that there probably are. With only a claim that errors sometimes occur, allowing that possibility doesn't make the original overconfident prediction.
- B
The conclusion points the other way. Aisha has found no errors and expects probably none in the remaining pages. Our argument predicts rain in the remaining week, rather than continued dryness. Similar subject matter and premises don't repair that opposite result.
- CCorrect
The journal issue corresponds to the summer; errors correspond to rainfall; the last two pages correspond to the final week. A general expectation about errors per issue is used to predict errors in the unexamined remainder because none have appeared yet. An average isn't identical to a statement about what usually happens, but both arguments make the same unsupported move from the overall pattern to the remaining portion of this particular case.
- D
This concerns several completed issues followed by a new issue. That would resemble using several past summers to forecast another summer, not predicting rain in the last week of an unfinished summer. It also expects the usual absence of errors to continue, rather than expecting a missing event to appear at the end.
- E
The whole issue has already been checked. Aisha's proposed explanation is that her checking was mistaken. The weather version would say the summer is over and the rainfall measurements must have missed some rain. Our argument instead accepts the report of no rain so far and predicts rain in time still remaining.
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