25. This LSAT error question tests your quantitative reasoning skills. In particular, be wary of "most" and the way it appears in the argument.

	P1: 75% (most) of problems cleared up w/in 50 weeks of therapy.
	_______________________
	C: 50 weeks of behavioral therapy sessions all that more than half (most) people need

The argument jumps from "therapy" to "behavioral therapy". And, it jumps from "most problems" to "most people", and, because of all the numbers thrown about in the argument, it's likely that the correct choice (and the incorrect choices) will address this. So, figure out what's going on beforehand in simple charts. The premise gives us this:

 

	Consistently cleared within 50 weeks	Not consistently cleared within 50
	45 (about 75%)	15

 

The conclusion only provides "most" people, which means greater than half . To make things easy, plug in '100', and thus "most" could equal 51 out of 100:

	Only need 50 weeks therapy	Do not only need 50 weeks therapy
	51 (most)	49

 

(A) is incorrect because the argument allows for the possibility that other problems take longer than 50 weeks to clear up.

(B) is correct because if one of the 15 problems in the "Not consistently cleared within 50" column affects more than half of all people, then more than half of all people will NOT only need 50 weeks of therapy.

(C) states "no one", which is extreme. (C) is wrong because the argument allows for the possibility that at least one person suffers from more than one of the 60 problems. If (C) stated "ignores the possibility that most people suffer from more than one of the problems", then (C) might be a correct choice.

(D) is incorrect because, although the author does do this, it is not a weakness in the argument, which concludes the behavioral therapy is all that is needed. Thus, whether some forms have never been proven effective is irrelevant.

(E) tries to attack the sampling assumption that the Therapist makes that the "large, diverse sample" represents all people. However, focus on "more". If the sample had more problems than the general population, then the argument is might be strengthened by (E) because if the the general population has fewer problems, then it's less likely that they have one of the 15 not consistently cleared within 50 weeks.

 

26. For this LSAT weaken question, ensure that you keep in mind the Researcher's exact conclusion (final sentence) and how narrow its scope is. Also, ensure that you seek a choice that weakens the Researcher's reasoning, not her method of conducting research to gain information on which to draw a conclusion. The author states "as great as", which in context essentially means "equal to". The author assumes that current genetic patterns absolutely determine ancestry. However, perhaps mice and guinea pigs did descend from a common ancestor, yet, over thousands of years the genetic patterns of the two species have developed differently.

(A) attempts to attack a sampling assumption that the author does not make. The conclusion is only about guinea pigs, not about all rodents.

(B) is the correct choice because it provides information that implies that current genetic structural similarity does not necessarily determine common ancestry.

(C) tries to attack the Researcher's methods for gaining information rather than her reasoning based on that information.

(D) provides a comparison between biological orders, which has no effect on the argument. If we put in specifics, (D) reads: "The common ancestor of rodents predates the common ancestor of carnivores".

(E) refers to similar body structures of rodents, which is irrelevant -- the author states "despite their similar physical form".