A recent study found that snoring, though not common in either group, is more common among smokers than among non-smokers. On the basis of this evidence, the author hypothesized that smoking by itself can induce snoring.

Which one of the following, if true, casts the most doubt on the author’s hypothesis?

[A]. Stress induces both snoring and smoking in certain individuals.
[B]. Obesity induces many individuals to smoke.
[C]. Most snorers do not smoke.
[D]. Most smokers do not snore.
[E]. Both smoking and snoring cause throat problems.
Answer: A

This stimulus presents a classic correlation-causation argument. A study found that smokers are more likely to snore than non-smokers, even though snoring is not particularly common in either group. From this, the author of the study concludes that smoking alone can cause snoring. Not so fast. All we know is that two conditions (smoking and snoring) are connected, and that isn’t enough to conclude that the first condition (smoking) causes the second (snoring). For all we know, the causal relationship could be the other way around—snoring could cause smoking. Although that seems somewhat unlikely in this case, nothing rules this possibility out. Alternatively, both conditions could be caused by some third factor (such as stress), as (A) suggests. Thus (A), if true, would explain how smoking and snoring are correlated without implying a causal relationship between them. So (A) weakens the argument by providing an alternative explanation of the author’s evidence.

(B) provides evidence concerning the causes of smoking, but this argument concerns the effects of smoking, and whether one of them is snoring. By ignoring smoking’s effects in general, and the issue of snoring in particular, (B) misses the point.

(C) and (D) both address the degree of correlation between smoking and snoring, whereas the author only addresses the question of whether any causal relationship exists. So (C) and (D) might be relevant to the question of the extent of the connection between smoking and snoring, but they are irrelevant to this argument, which only claims that smoking can lead to snoring, at least some of the time. Furthermore, choice (D) is practically inferable from the evidence stating that snoring is not common among either smokers or non-smokers.

(E) has it backwards. Whereas the credited response suggests that a third condition could cause both smoking and snoring, (E) suggests that both smoking and snoring cause a third condition. This has nothing to do with the author’s conclusion that smoking can cause snoring, so it in no way weakens the argument.

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