PrepTest 104, Section 1, Question 23
Research indicates that 90 percent of extreme insomniacs(22 more words not shown)
Which one of the following(9 more words not shown)
- AIt fails to(23 more words not shown)
- BIt fails to(14 more words not shown)
- CIt relies on(18 more words not shown)
- DIt draws an(17 more words not shown)
- EIt presumes without(7 more words not shown)
Reading the argument
Lots of insomniacs drink coffee. That doesn't mean lots of coffee drinkers are insomniacs.
The 90 percent figure starts with people who have extreme insomnia and asks how many drink a lot of coffee. Tom enters from the other side: we know he drinks a lot of coffee, and we're trying to work out whether he has extreme insomnia. There could be a huge number of heavy coffee drinkers who sleep without that problem. The original statistic doesn't tell us how large that group is.
For example, imagine 100 extreme insomniacs, 90 of whom drink a lot of coffee. Now add 9,910 other heavy coffee drinkers who are not extreme insomniacs. The stated 90 percent remains true, but only 90 of the 10,000 heavy coffee drinkers are extreme insomniacs. That's less than one percent. These are hypothetical numbers, but they show why the conclusion about Tom doesn't follow.
What we’re looking for
The flaw question should point to this switch between groups. We need the rate of extreme insomnia among heavy coffee drinkers. We were given the rate of heavy coffee drinking among extreme insomniacs.
Working the answers
- A
Where did that 10 percent come from? The remaining 10 percent in the research are extreme insomniacs who do not drink large amounts of coffee. They are not heavy coffee drinkers who lack extreme insomnia. A makes the same group switch as the argument.
It also treats the possibility of an exception as though that alone defeats a claim of likelihood. A likely outcome can have exceptions. Here, the deeper problem is that we have not established the relevant likelihood for Tom's group at all.
- B
We aren't trying to identify the causes of insomnia. The question is whether Tom's coffee drinking gives us a good reason to think he has extreme insomnia. Knowing there may be other causes does not resolve the missing information about the heavy-coffee-drinking group.
- CCorrect
This asks for exactly the rate the argument needs and does not have: among people who drink a lot of coffee, how common is extreme insomnia? The evidence instead gives us a statistic about coffee drinking within the extreme-insomnia group. Without knowing how the coffee-drinking group is made up, we cannot transfer that 90 percent figure to Tom.
- D
Applying information about a group to one of its members is not automatically a mistake. If we had been told that 90 percent of heavy coffee drinkers were extreme insomniacs, that would support a likely conclusion about a heavy coffee drinker such as Tom. The problem is that our statistic starts with a different group. D points to the general-to-individual move while missing the particular mismatch that makes it fail here.
- E
The argument doesn't say coffee always causes insomnia. Its conclusion is about what Tom is likely to be, and its evidence describes a relationship between two groups, not an established cause. E replaces that reasoning with an absolute causal claim the author never makes.
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