Correlations and Causality on the GRE Part III

Comic via xkcd

The previous blog article in this series provided an example of a short Reading Comprehension passage containing causal reasoning, and followed it with a Weaken question. Today we discuss the correct and incorrect answer choices for that question.

Let’s start by revisiting the passage and question:

Advertisement: Pump3D is a nutritional supplement that can greatly reduce athletes’fatigue after anaerobic exercise. This was shown by a study investigating the relationship between fatigue and high doses of guarana extract—the main ingredient in Pump3D—which showed that people who regularly take guarana extract supplements have a significantly lower level of fatigue after anaerobic exercise than people who do not take the supplement.

Which of the following, if true, would most weaken the argument in the advertisement?

(A) Guarana extract supplements tend to reduce slightly athletes’ endurance and stamina during aerobic exercise.

(B) People who regularly consume guarana extract supplements tend to be professional athletes, who rarely experience fatigue after exercise.

(C) Guarana extract is only one of several ingredients in Pump3D.

(D) The reduction in fatigue due to the consumption of guarana extract is no greater than the reduction due to other nutritional supplements.

(E) Guarana extract taken in the form of supplements has a different effect on the body than does guarana extract taken in food.

Answer and Analysis

Since the key to weakening an argument is to focus on the conclusion, it is essential to break down the argument in to its component pieces. The conclusion is the first sentence in the stimulus, in support of which the author describes the results of one particular study (“this is shown by” is a premise indicator):

Premise: People who regularly take guarana extract supplements have a significantly lower level of fatigue after anaerobic exercise than people who do not take the supplement.

Premise: Guarana extract is the main ingredient in Pump3D.

Conclusion: Pump3D can greatly reduce athletes’ fatigue after anaerobic exercise.

The advertisement’s conclusion is based on a questionable assumption, since the study only establishes a correlation between lower level of fatigue and guarana supplementation. While there are many ways of attacking the conclusion, most will require undermining the reliability of the study or experiment. First, was the study properly controlled? In other words, are we sure that the subjects who did not take guarana supplementation had the same baseline fitness level as those who did? Second, do we have any reason to suspect that taking supplements would have a placebo effect that would lead the study group to overestimate the degree to which their fatigue has been reduced? Third, is it possible that the researchers themselves are biased in favor of discovering the sought-after effect?

We can also weaken the argument by questioning whether the causal relationship can be explained by a third, independent factor. What if people who typically experience lower level of fatigue because of a third, unrelated factor (above-average fitness level, other nutritional supplements, etc.) also tend to take Pump3D? This would imply an alternate cause for the observed effect, and agrees most closely with answer choice (B).

• Answer choice (A) is incorrect, because the scope of the argument is limited to the effects of guarana supplementation on anaerobic exercise alone. The effect on aerobic exercise would only be relevant if the conclusion extended to include other forms of exercise (or exercise in general). Be aware of argument scope, and avoid weakening a more general version of the conclusion than the one presented.
• Answer choice (B) is the correct answer choice. Recall that the advertisement’s conclusion is based upon a questionable assumption, since the study only establishes a correlation between lower level of fatigue and guarana supplementation. If the subjects who took guarana extract did not have the same baseline fitness level as those who did, then the study was not properly controlled. This, in turn, opens up the possibility of an alternative interpretation for the observed correlation: rather than arguing that guarana reduces fatigue, it is now possible that some third factor—being a professional athlete, for instance—is causally related both to experiencing lower levels of fatigue (professional athletes rarely do), and to the consumption of guarana extract (professional athletes regularly consume it).
• Answer choice (C) may seem attractive, because we know that guarana extract is only one of several ingredients in Pump3D. However, we cannot weaken what we do not know: it is entirely possible that the other ingredients in Pump3D are inactive, or that they only intensify the stimulating effects of guarana.
• Answer choice (D) is perhaps the most attractive incorrect answer choice. If the reduction in fatigue due to the consumption of guarana is no greater than the reduction due to other nutritional supplements, this suggests that guarana is not the only (or the best) way to reduce fatigue. Other nutritional supplements work just as well. This does not weaken the conclusion, however, which only argues that a nutritional supplement containing guarana can greatly reduce athletes’ fatigue. The author never claimed that Pump3D would be the best (or the only) way to reduce fatigue. Even if some other supplement is just as good (or better), as long as Pump3D works as advertised the argument still stands.
• Answer choice (E) is incorrect, because it plays upon the commonly held belief that supplements are not the “real thing.” Read closely! The study examined subjects who took nutritional supplements containing guarana extract, making it reasonable to draw a conclusion about the effects of a similar supplement, such as Pump3D. Had the study examined the effects of guarana when taken with food, this answer choice would have been a reasonable objection to the conclusion.

Click here to read Part I and Part II of this series.