PrepTest 108, Section 2, Question 2
John: For 40 years, fluoride has been added(63 more words not shown)
Which one of the following(7 more words not shown)
- AJohn does not(11 more words not shown)
- BJohn does not(7 more words not shown)
- CJohn does not(20 more words not shown)
- DJohn does not(11 more words not shown)
- EJohn does not(9 more words not shown)
Reading the argument
Rats developed bone cancer. Does that tell John what will happen to him?
I'd first want to know more about the experiment. How much fluoridated water did the rats receive? Would people be exposed in the same way? We also see a difference between the male and female rats. Those facts make me cautious about carrying the result straight over to John.
But his conclusion makes an even bigger leap. He says that stopping will let him be sure he won't develop bone cancer.
What if there's another way to get it? Imagine a person who never drinks this water but develops the disease for an unrelated reason. Nothing in the study rules that possibility out. Even if we granted John that the water increases his risk, removing that one possible cause wouldn't establish that every cause had been removed.
What we’re looking for
For the strongest criticism, I'd look for exactly that problem with his certainty. We aren't being asked whether avoiding the water has any value. We're questioning whether it guarantees John the protection he claims.
Working the answers
- A
John's conclusion concerns his own risk. Information about other people might matter to a broader public-health decision, but he isn't making that broader decision here. Failing to discuss everyone else's experience doesn't identify why his claimed guarantee about himself is unsupported.
- B
Other diseases are a different question. The water might cause some other illness or cause no other illness at all, and we'd still need to know whether John could develop bone cancer without drinking it. B doesn't address that possibility.
- C
A brief interruption in fluoridation doesn't resolve the concern. Whether the water was treated every day of those 40 years or occasionally left untreated tells us nothing about other ways John could develop bone cancer after he stops drinking it. The flaw isn't an incomplete day-by-day history of the water supply.
- D
Possible benefits of the water could matter if we were weighing every advantage and disadvantage of drinking it. But showing that something has benefits wouldn't establish whether eliminating it prevents bone cancer. John hasn't merely concluded that the disadvantages outweigh the benefits; he has claimed certainty about avoiding a particular disease.
- ECorrect
John overlooks other possible causes of bone cancer. Evidence that one thing causes a disease doesn't establish that it is the only thing that can cause it. Even accepting the study and its relevance to John, we could at most remove the risk attributed to that source. Without ruling out other causes, he cannot conclude that stopping makes him safe from the disease.
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