LSAT PrepTest 20 Section 4 Question 7 Explanation | Logical Reasoning

I didn't write the following blog post. It was already on the blog when I took over the URL. The following blog post may contain mistakes. -Steve

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Here's a Logical Reasoning question from the October 1996 LSAT.

Here's a chain of phrases (this is for instructional purposes; when you're really taking tests, just underline/bold a few key words like I did):

1/50 die of disease --> vaccination kills 1/5000 --> vaccination should be used

The reason why we'd want to use a vaccine on every dog, of course, is that it does more good than harm in the end. Say we have 5,000 dogs; 2% of the dogs that get a disease will die, and 1/5000 who gets the vaccination will die. What do we need to know to decide whether the vaccination of all 5000 dogs does more good than harm (results in fewer deaths)?

We need to know how often dogs get the disease! If 1 in two million dogs gets the disease, it won't be worth it to use a vaccination that kills 1/5000, even if 1/2 the dogs with the disease die (1 dog dies out of a million compared to 1/5000 with the vaccinations). But if 50% of dogs get it and half of those die, that means 1250 (1/2 of 1/2 of 5000) out of 5000 die, compared to 1/5000 with the vaccinations.

So, we can pre-phrase an answer; we'd like to know something like "the chance a dog will get the disease." We see that E fits this well right away, but let's look at the other choices quickly (if you were running out of time, you could probably skip this, but if you have time, do it):

A) Out of scope. This doesn't help us evaluate whether fewer dogs will die under the vaccination program versus without it. The number of dogs that die from other causes is the same in either case (since we're talking only about this disease, not fixing other causes of dogs dying).

B) Out of scope again. We're talking about the effect on dogs and what's best for them. It would be nice if it worked on other pets, but whether or not it does doesn't matter for dogs, and that's the decision we're making (should we vaccinate all dogs?).

C) Wrong for the exact same reason as A.

D) Wrong for the exact same reason as A and C.

E) Correct.

Remember:

1) Use a chain of phrases (underlining normally does this, but I write it out here to make things clear) and a pre-phrase (which also isn't written down on the real test, usually, although I've written down a word or two occasionally when the pre-phrase is kind of complex). This makes you more accurate and fast in finding the right answer among a bunch of wrong ones.

2) Get rid of choices outside of the scope of the passage. If the choice doesn't bear on the facts of the passage, in this case, that obviously isn't the piece of information we need!