PrepTest 155, Section 2, Question 22
Merle: Usually when I insert a dollar bill(47 more words not shown)
Which one of the following(14 more words not shown)
- AEveryone who has(31 more words not shown)
- BSome people who(30 more words not shown)
- CMany people who(35 more words not shown)
- DMost people who(28 more words not shown)
- EMost people who(31 more words not shown)
Reading the argument
What if Merle turns the outlet on just before using the machine and switches it off afterward?
Then the machine could squeak whenever Merle uses it while the outlet spends most of its time turned off. We don't have to claim that's what happens. It's enough to see that the observations during use don't tell us what happens during all the hours when Merle isn't using the machine.
The first premise concerns a limited set of occasions: usually, when Merle inserts a dollar, the machine squeaks. Squeaking requires the outlet to be on. So the outlet is usually on during those dollar-inserting occasions. The conclusion drops that restriction and says it is usually on in general.
For the parallel flaw, look for the same expansion from a selected group to a broader group. Keep the other relationship straight too: the observed feature requires another feature. We infer that the second feature is common in the selected group, then improperly extend that claim beyond the group.
What we’re looking for
Don't stop at calling this a biased sample. To recognize the same mistake elsewhere, we need to track which group is being counted and which property the conclusion assigns to it.
Working the answers
- A
Having a vivid imagination is enough to be disturbed by the plot, according to this answer. But A then assumes that the disturbed readers must have that imagination. Someone without it could find the plot disturbing for another reason. That's a different mistake: treating a way to get a reaction as the only way to get it. Merle's second premise expressly makes the outlet being on a requirement for the squeak.
- B
Some readers were disturbed, and anyone without a vivid imagination would not be disturbed. Those disturbed readers must therefore have vivid imaginations. That establishes that some people have them. B doesn't turn a fact about the share of readers into a claim about the share of all people; its limited existence claim follows.
- C
Many readers were disturbed, and all disturbed readers have vivid imaginations. Therefore, many readers have vivid imaginations. The conclusion stays inside the reader group. It makes the inference Merle could safely make about the outlet during the times the machine is used, rather than extending it to all times.
- DCorrect
The readers correspond to the occasions when Merle inserts a dollar. Finding the plot disturbing corresponds to the machine squeaking. Having a vivid imagination corresponds to the outlet being turned on: it is required for the observed response.
Most readers are disturbed, so most readers must have vivid imaginations. But D concludes that most people have vivid imaginations. Maybe people with vivid imaginations are especially likely to choose this novel. The readers could be an unusual group, just as the moments when Merle uses the machine could be unusual times for the outlet. Both arguments drop the restriction that made the initial claim supportable.
- E
The first two statements match D, but the conclusion changes the group to people with vivid imaginations and claims that most of them find the plot disturbing. They might never have read the novel. Having the required imagination doesn't make the reaction common among everyone who has that imagination.
Map it back to the machine: E is closer to inferring that the machine usually squeaks when the outlet is on. Merle instead concludes that the outlet is usually on overall. The property being asserted and the group being counted are both different. That's why D, not E, matches.
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