Logical reasoning PrepTest 127 · Section 3 · Question 20

Incorrect. The stimulus has "directionality"—vitamin C doesn't just affect health, it improves it. As such, a neutral answer like this one doesn't affect the argument. While the effects of vitamin C may be different based on different sources, without knowing what "direction" that difference is (improve, worsen), it's impossible to know how this plays into the argument. Additionally, there's nothing in the stimulus setting a baseline for natural vitamin C, so this answer is also out of scope.

Incorrect. Other dietary changes improving health doesn't affect whether or not vitamin C does. Even if other changes might affect the average, it's already been established that those who take high doses of vitamin C have "lower than average" levels of heart disease, so the actual average is immaterial.

Incorrect. Something working better doesn't affect whether something works.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Study - Those who take large doses of vitamin C have lower levels of heart disease
Conclusion - People who take vitamin C tend to be healthier than average

Answer Anticipation:
Be careful here! This is a Weaken question, and so there's a good chance of a correlation/causation flaw. And this argument certainly feels as if it has a causal conclusion, it actually doesn't. Note the language—it doesn't say that vitamin C makes people healthier, just that they tend to be healthier. That's a correlation, so this argument jumps from a correlation to a correlation—that's not a correlation/causation flaw.

So what is wrong with the argument?

Well, since it is based on a study, we can still consider the other common flaw associated with it—a sampling flaw. Here, the study looked at those who took vitamin C in high doses and saw that they had a "significantly lower than average" amount of heart disease. That's comparing a group with a specific characteristic to the average, which can yield a valid conclusion. It's not a perfect sample (there's no information about how it was formed or how large it is), but there's no information in the stimulus that actively calls it into question. There might be a jump between high doses of vitamin C and people who take any vitamin C supplements, but because it's talking averages here, that group as a whole would have lower risks if a subset of the group did and the rest had just the average risk.

This can leave you at a bit of a loss as to the flaw! If you're having trouble spotting the gap, it's always good to start looking at the conclusion. What's it about? Taking vitamin C, and overall health. The premises do discuss taking vitamin C, but they only discuss a part of overall health—heart disease. While it's true that, all else equal, a group with lower levels of heart disease will be healthier than average, there's no indication that all else is equal. Vitamin C may improve heart health while worsening other types of health—and if that's the case, then it's questionable whether they're "healthier than average." Let's find an answer raising the possibility that vitamin C supplements can result in other illnesses.

Answer Explanation:
This answer brings up another effect of high doses of vitamin C on health—and it's a negative one. If vitamin C can contribute to increasing risk of infectious disease (by lowering resistance), then it might not improve overall health even if it reduces risk of heart disease. This answer addresses the jump from heart disease rates to overall health and weakens the argument.

Key Takeaway:
Be careful with applying rules of thumb! Here, it would have been easy to mistakenly consider this a correlation/causation flaw because it feels causal, and we know that a lot of Weaken questions—especially those reliant on a study—have that flaw. That would have likely prevented you from seeing the shift from discussing one aspect of health to overall health, which was the flaw that we needed to spot in order to get to the correct answer.

Incorrect. This answer aligns with the argument—if vitamin C lowers risk of both heart attack and cancer, then that's more evidence people who take supplements will be healthier than average.