Practice GMAT Critical Reasoning Question

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After studying a random sample of 1024 individuals who had smoked daily for at least three years and comparing the results of this study with the results of a study of 1024 individuals who had never smoked, a group of researchers concluded that habitual smoking causes increased difficulties in concentrating.

Which of the following, if true, most severely weakens the researchers' conclusion?

A)	The addiction to smoking and the cravings this addiction engenders is often on the mind of habitual smokers.
B)	Some non-smokers with attention deficit disorder (ADD), which causes an inability to concentrate, display even less ability to concentrate than some smokers.
C)	A separate research study found that smokers and non-smokers exhibited statistically significant differences in their incarceration rates.
D)	After developing a severe addiction to smoking for fifteen years, the ability of many individuals to concentrate is decreased.
E)	A separate research study found that individuals with preexisting attention and concentration disorders exhibited significantly higher rates of trying cigarettes and subsequently becoming addicted to smoking.

Correct Answer: E

The researchers' conclusion is: "habitual smoking causes increased difficulties in concentrating"

The group of researchers confuses correlation with causation. In other words, the group concludes that a correlation between smoking and an inability to concentrate implies that smoking causes an inability to concentrate. Perhaps it is the other way around and an inability to concentrate causes individuals to become distracted and take up smoking. Evidence to support this counter theory would weaken the researchers' conclusion.

This answer strengthens (not weakens) the original argument as it provides an explanation for how habitual smoking increases difficulties in concentrating after becoming hooked.
The answer deals only with some ADD non-smokers and some smokers so it does not provide any solid evidence and justification to weaken or reject the original argument. Further, it is not logical to compare the concentration ability of individuals with an attention deficit condition to other individuals who do not have an attention deficit condition.
The incarceration rate is irrelevant in determining the relationship between concentration and smoking. Whether smokers are incarcerated at higher or lower rates does not enable us to strengthen or weaken the causal relationship between smoking and concentration proposed in the original argument.
This strengthens (not weakens) the original argument as it intensifies the causal relationship between smoking and subsequent difficulties concentrating.
This additional study pinpoints that individuals with preexisting (or already established) concentration difficulties subsequently became addicted to smoking. This pinpoints that the smoking could not have caused the attention and concentration difficulties (as these difficulties already existed prior to becoming addicted).

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