PrepTest 153, Section 3, Question 11
Studies have shown that those who take daily(35 more words not shown)
The questionable reasoning in the(12 more words not shown)
- ATo be classified(37 more words not shown)
- BEveryone who ate(26 more words not shown)
- CLaboratory rats given(36 more words not shown)
- DPeople who train(21 more words not shown)
- EAutomobile engines that(29 more words not shown)
Reading the argument
People who take vitamin C are less likely to catch colds. Is the vitamin doing the work, or could those people be healthier in other ways? That's a question I'd ask if the author claimed that vitamin C caused the difference. But the conclusion makes a different move: someone has a cold, so that person probably doesn't take vitamin C.
How many people take it?
Imagine 10,000 people, with 99 percent taking a daily dose. That gives us 9,900 users and only 100 nonusers. Suppose 10 percent of the users get colds, compared with 20 percent of the nonusers. Users really are less likely to get colds, just as the premise says.
Now count the people with colds: 990 users and 20 nonusers. Almost everyone with a cold in this example takes vitamin C. The users' lower rate doesn't overcome the fact that there are so many more users to begin with. We could accept the study's comparison and still reject the conclusion about which group a sick person probably belongs to.
What we’re looking for
For the parallel flaw, follow that move. One group is less likely to have a problem. Then, on seeing the problem, the author concludes that the individual probably isn't in that group. We need another argument that ignores how large the groups might be, not just another argument that makes some mistake about health.
Working the answers
- A
This starts with a wind-speed requirement for being classified as a hurricane and concludes that storms with winds around that threshold are not hurricanes. Our argument does not define colds by a required feature. It compares the likelihood of colds in two groups, then guesses group membership from the outcome. A lacks that probability comparison and that backward guess.
- B
The premise says everyone who ate the tuna salad became ill. The conclusion then extends the illness prediction to people who ate other dishes. That is a jump from one food group to different food groups. It doesn't start with a lower illness rate in one group and use someone's illness to guess which group they came from.
- C
This moves from results in rats to a prediction about humans. The diet comparison may make the start sound similar, but the conclusion changes populations. The vitamin argument stays within its population and reasons from having a cold to probably not taking the vitamin. A rat-to-human prediction is a different error.
- D
Training can help produce skill without being the only contributor. Perhaps experience, judgment, or other abilities matter too. D improperly gives hard work all the credit. It doesn't identify a player with a problem and infer that the player probably lacks training, which would resemble the original move.
- ECorrect
Regular oil changes match daily vitamin C. Engine problems match colds. Regularly maintained engines are less likely to have problems, but a car with a problem is then said probably not to have received that maintenance.
Use the same population concern: if nearly every car gets regular oil changes, that large group could still supply most of the cars with engine trouble, despite its lower trouble rate. E makes exactly the unsupported move our numerical example exposed.
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