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Here is a Logical Reasoning question from the June 2003 LSAT. It addresses correlation vs. causation, which comes up at least once on every exam.
We're looking for a choice that exposes a flaw in the reasoning - a gap between the conclusion and the premises.
Breakdown of the argument:
Premise 1: Hard water contains more magnesium than soft water.
Premise 2: People being treated for heart disease, stroke, and hypertension have lower levels of magnesium in their blood.
Conclusion: Therefore, those who drink mostly soft water have a higher risk of these diseases.
(Note: the 2nd premise comes after the argument's conclusion. In other LR questions, the conclusion might the first sentence.)
Remember: correlation (a relationship between two events) doesn't necessarily imply causation (that one event causes another.) For example, lets say that A occurs whenever B does. This doesn't mean that A causes B.
However, this is not necessarily true. Maybe B causes A, or both are caused by a third factor, C. Alternatively, it might be a coincidence.
In this case, the author mistakenly assumes lower levels of magnesium cause the diseases noted in the stimulus. (That’s why the author concludes that drinking soft water, with less magnesium, will increase these diseases.)
However, this isn’t necessarily true. The diseases themselves could cause the decline in magnesium in the blood. This would mean the actual intake of magnesium might have no effect on whether or not you develop the diseases.
Choice E is the correct answer because it highlights this issue. If compounds used to treat these diseases lead to reduced magnesium, then this reduction is probably an indirect effect of the diseases, not a cause, and the conclusion would be incorrect.
Choice C doesn’t weaken the argument. In fact, if we accept the author’s theory (decrease in magnesium causes the diseases), the decrease with age would make consumption of magnesium more important.
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