Akela wrote:
To cut costs, a high school modified its airconditioning system to increase its efficiency. The modified system, however, caused the humidity in the school air to decrease by 18 percent. Twenty-four hours after the decrease in air humidity, a 25 percent increase in the number of visits to the school nurse was reported. This shows that a decrease in humidity can make people ill.
The argument depends on assuming which one of the following?
(A) At least some of the visits to the school nurse after the system was modified were due to illness.
(B) Most of the students at the high school suffered from the decrease in air humidity.
(C) It takes 24 hours after a person is infected with a virus for that person to exhibit symptoms.
(D) A decrease of 18 percent in air humidity causes an increase of 25 percent in one’s probability of becoming ill.
(E) Modifying the air-conditioning system proved to be an ineffective way to cut costs.
Source: LSAT
EXPLANATION FROM Fox LSAT
How do we know there wasn’t a rampant gonorrhea outbreak among the students at Skank Unified High that same day? Or how do we know that the AC didn’t happen to get installed right before final exams, when everyone invents bogus illnesses and goes to the nurse? Or how do we know the school bus didn’t get in a 50-car pileup, causing massive injuries among the students and requiring nurse visits?
Yes, the humidity dropped. Yes, nurse visits increased. But we have no proof that one thing caused the other.
Correlation does not prove causation.
We’re asked to identify an assumption. I think it will be something that points out that
correlation doesn’t necessarily prove causation.
A) Well yeah. If this isn’t true, it becomes, “None of the visits to the school nurse after the AC system was modified were due to illness.” If that’s true, then how can it be true that the AC change
caused illness? This must be true in order for the argument to make sense. Therefore it is a Necessary Assumption of the argument.
B) No, it’s not necessary that “most” students suffered in order for the argument to make sense. Even if “less than half” of the students got sick, the illness could still have been caused by the AC. So this isn’t necessary.
C) Who said anything at all about viruses? There is no way this was assumed by the speaker. Nobody is even talking about it. (There are all sorts of illnesses that have nothing to do with viruses.)
D) This strengthens the argument. But it’s so specific that there’s no way it’s a
necessary support for the argument. What if a decrease of 18 percent in air humidity causes an increase of 2500 percent in one’s probability of getting ill? If that’s true, then D is false. But the argument would still make sense.
On a Necessary Assumption question, the opposite of the correct answer choice should kill the argument. That’s how you know something is necessary! Example: Oxygen is necessary, because without it you die. Food is also necessary. But pizza is not necessary, because without pizza you can still survive.
E) Costs are irrelevant.
Our answer is A.