Logical reasoning PrepTest 155 · Section 1 · Question 10

This has no effect on the conclusion. Perhaps if we knew that U.S. and European kids used a comparable amount of fluoridated toothpaste, that would present a similarity between the two groups, further strengthening the argument's comparison. However, the answer choice doesn't quite say that. Just because fluoride in toothpaste is widely available in both places doesn't mean U.S. and European children use a similar amount of fluoridated toothpaste. In fact, this conclusion allows the possibility that children in Europe use far more fluoridated toothpaste than children in the U.S., which could weaken the conclusion.

Generally, to strengthen a comparison, we need an answer choice that provides information about both groups. This answer choice only provides information about the U.S. Nearly all U.S. dentists may use fluoride, but what about European dentists? Do they apply fluoride? And how often do U.S. and European children go to the dentist, for that matter? Without knowing more information about European children and dentists, we can't say that this answer choice strengthens the comparison made in this argument.

This answer choice presents a similarity between the U.S. and Europe, so it's a tempting answer choice based on our anticipation. However, just because dental hygiene is taught in both U.S. and European classrooms doesn't mean that it's taught in the same way or that the children internalize the same lessons. If U.S. children are taught to only brush their teeth once a week and floss biannually, while European children are taught to brush three times a day and floss daily, this could weaken the comparison made in this argument.

This weakens the argument. Kids in Europe are getting more dental care; maybe that's why their teeth are better. Rather than presenting a relevant similarity between the U.S. and European children, this presents a difference that further strains the comparison made in this argument.

Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen

Stimulus Summary:
Five studies show that kids in Europe, where the water isn't fluoridated, have less tooth decay than kids in the USA, where water is fluoridated. So fluoridation of water doesn't prevent tooth decay.

Strategy Overview:

1. Argument or facts? Always argument, so identify premises and conclusions of argument
2. Anticipate why the premises are not enough to show that the conclusion is true
   1. Causal argument? Look for answer choice that:
      1. Rules out possible alternate cause
      2. Shows same cause, same effect
      3. Shows no cause, no effect
      4. Shows the cause and effect are not reversed
   2. If not causal argument, anticipate ways to fix problem with argument
3. Use anticipations to select the answer choice that, if true, would strengthen argument

Answer Anticipation:
On Strengthen questions, the correct answer usually fixes a flaw in the argument. This is why we should try to identify a problem with the argument first on these questions. Once we identify the problem, we're better equipped to pick an answer choice that fixes that problem.

This particular argument doesn't seem to be causal (notice how the conclusion asserts that fluoride doesn't seem to cause a lower incidence of tooth decay), so we should look for a different problem. Another widespread issue on Strengthen questions is a comparison fallacy. And, indeed, this argument rests on a comparison. It compares the rate of dental issues in European kids and U.S. kids. The problem with comparing two groups is that significant differences in the two groups could make the comparison unfair or misleading.

There could be many factors unrelated to fluoride in water that might increase dental problems in U.S. kids. For instance, what if U.S. kids eat nothing but sticky, cavity-causing sweets while European kids eat a healthy and balanced diet? Could we conclude that fluoride in water plays no role in preventing dental issues if that were the case? Probably not. Perhaps the U.S. diet is so bad that even with fluoride's help, there are still more pediatric dental issues in the U.S. than in Europe. There could be many other complicating factors as well: access to dental care, norms about dental hygiene, childhood poverty, genetics — the list goes on. The correct answer choice will probably eliminate one of these potential differences, making the argument a more level, fair comparison.

Answer Choice Explanation:
Finally, something that strengthens the argument, and exactly how we expected it to: by presenting a relevant similarity between the European and U.S. kids. If both groups' diet is roughly the same, then we know a difference in diet isn't producing the difference in the rate of dental issues. Imagine if the opposite of this answer choice were true: if U.S. children's diets were much worse for their teeth than European ones, we couldn't say that fluoride doesn't prevent tooth decay. If the opposite of this answer choice were true, the U.S. diet might just result in more dental issues, even if fluoride is helping prevent even more dental problems. This answer choice blocks this potentially argument-ruining difference and thus strengthens the argument.

Key Takeaway:
We can think of this comparison as "uncontrolled." Remember learning in biology class that scientific studies need a control group to determine if changing one variable results in a meaningful change? And without a control group, we can't be certain which variable produced the change? The same idea is at work here. Yes, there's a difference in whether water is fluoridated in the U.S. and Europe. But there could be many other differences leading to the difference in dental problems. And these other potential differences make the comparison between the U.S. and Europe uncontrolled. The correct answer removed one of these possible differences, making the comparison more controlled and thus more persuasive. Uncontrolled comparisons are rampant on the LSAT, and spotting them will help you improve your score immensely.