Bunuel wrote:
A study of more than 5,000 people showed that those who lived at high latitudes, where UV exposure is minimal, had higher cancer rates than did those who lived near the equator, where UV exposure is, on average, far greater. One of the effects of UV radiation on humans is that it stimulates vitamin D manufacture in exposed skin. These results are consistent with other studies demonstrating that cancer patients often have lower than average vitamin D levels. Therefore, high blood levels of vitamin D reduce the risk of cancer in humans.
The answer to which of the following questions would be most useful to know in order to evaluate the validity of the argument’s conclusion?
(A) Does seasonal affective disorder, experienced more frequently by those living at high latitudes than by those living at the equator, predispose its sufferers to cancer?
(B) Do high blood levels of vitamin D reduce the incidence of the common cold?
(C) Can blood levels of vitamin D be raised through supplementation, thus reducing the risk of sunburn due to UV
exposure?
(D) Does exposure to UV radiation prevent cancer in other mammals, such as laboratory rats?
(E) Are wintertime low vitamin D levels responsible for the flu season phenomenon?
Official Explanation:
A
Step 1: Identify the Question TypeThe word “evaluate” indicates that this is an Evaluation question. The correct answer will contain a question relevant to the strength of the conclusion. Like Assumption, Strengthen, Weaken, and Flaw questions, Evaluation questions are built around assumptions in the stimuli.
Step 2: Untangle the StimulusBegin by identifying the conclusion, evidence, and assumption(s). The conclusion is signaled by the word “Therefore”: high vitamin D levels reduce human cancer risk. The evidence comes in two main parts. First, the sun stimulates vitamin D production. People who live near the equator, where it’s sunny, have lower cancer rates than do people who live in high latitudes and get less sun. Second, people suffering from cancer have unusually low vitamin D levels. The author makes several assumptions here. One is that people living at higher latitudes have lower blood vitamin D levels. We know that people at high latitudes get less sun—but it’s possible that their UV-related vitamin D deficit is offset by some other factor. The next assumption is that differing vitamin D levels, if in fact they do differ between people at high and low latitudes, contribute to the differing cancer rates in these populations. Third, the author assumes that the low vitamin D
levels observed in cancer patients are not an effect of the cancer or of treatment for the cancer. Finally, the author assumes that there are no other differences between people in high latitudes and people in low latitudes that could affect their cancer risk.
Step 3: Predict the AnswerWhenever an argument includes a causal conclusion (X caused Y), consider two possibilities: an alternative cause (Z caused Y) or a reversal of causality (Y caused X). In this case, both possibilities could be relevant. To assess the strength of the argument, we would want to know, first, whether some factor unrelated to vitamin D could account for the difference in cancer rates between people at high and low latitudes (that’s an alternative cause). Second, we would want to know whether the low levels of vitamin D observed in cancer sufferers were a cause of the cancer, as opposed to an effect of the cancer (that’s a reversal of causality). An answer choice that addresses either of these two issues is correct.
Step 4: Evaluate the Choices(A) matches our prediction and is the correct answer. If seasonal affective disorder does predispose humans to cancer, the conclusion that a low vitamin D level is responsible is greatly weakened. On the other hand, if seasonal affective disorder has no impact on cancer rates, then an alternative cause is ruled out, and the conclusion that vitamin D controls cancer risk is strengthened. (B) is outside the scope of the argument, which concerns cancer risk, not the risk of contracting a common cold. The method of raising vitamin D levels mentioned in (C) is likewise outside the scope, as are the “other mammals” mentioned in (D) and the “flu season phenomenon” mentioned in (E).
Choice (A) is the correct answer.