PT 27, Section 1, Question 23

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PT 27, Section 1, Question 23

Postby msw099 » Fri Feb 19, 2010 3:52 am

Hi All:

Can someone please explain why C is correct? Thank you in advance!!

Cambridge LSAT
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Re: PT 27, Section 1, Question 23

Postby Cambridge LSAT » Fri Feb 19, 2010 4:43 am

The speaker mistakenly infers 90 percent overlap in both directions. We don't know the percentage of big coffee drinkers who are extreme insomniacs. For example, consider this hypothetical case:
100 extreme insomniacs
90 of whom consume large amounts of coffee
1000 people total consume large amounts of coffee

In this case, if Tom were one of the 1000, would it be quite likely that he's one of the 90?

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Re: PT 27, Section 1, Question 23

Postby Shrimps » Fri Feb 19, 2010 12:55 pm

The reasoning of the argument: I -most-> C, therefore C -most-> I (I - insomniacs, C - heavy coffee drinkers). A very basic flaw.
(A) plays along and is based on the assumption that the reasoning is true.
(B) does the same.
(D) is not a flaw. We CAN draw conclusions about a person based on the characteristics of the group he belongs to, with the usual caveats (if most heavy coffee drinkers were, in fact, extreme insomniacs, then we would be justified in concluding that Tom is "quite likely.. an extreme insomniac").
(E) two shifts in scope (from "a lot of coffee" to just "drinking coffee"), and an absolute (ALWAYS) which wasn't in the stem (it only said 90%). Plus, of course, it also plays along and assumes the reasoning is not faulty.

(C) is the only answer choice that is not faulty. I.E. it specifically says that nothing in the statement "90% of extreme insomniacs drink lots of coffee" tells us about heavy coffee drinkers, except that SOME (could be just just a tiny fraction) of heavy coffee drinkers are insomniacs.

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