The prairie vole, a small North American grassland rodent, breeds year-round, and a group of voles living together consists primarily of an extended family, often including two or more litters. Voles commonly live in large groups from late autumn through winter; from spring through early autumn, however, most voles live in far smaller groups. The seasonal variation in group size can probably be explained by a seasonal variation in mortality among young voles.

Which of the following, if true, provides the strongest support for the explanation offered?

a. it is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles.
b. prairie vole populations vary dramatically in size from year to year
c. the prairie vole subsists primarily on broad-leaved plants that are abundant only in spring.
d. winters in the prarie voles' habitat are often harsh, with temperatures that drop well below freezing.
e. snakes, a major predator of young prarie voles, are active only from spring through early autumn.

My logic is that variation in young mortality -> group size. Doesn't a directly address this link by showing that it's the group size variation is attributed to young voles? I felt that e simply states a correlation ( trap answer), but doesn't really strengthen the direct link.

It provides the strongest support among all the choices, though and that is what we are asked. In addition, the discription of the snakes as a major predator of young prarie voles active only from spring through early autumn strongly sugests mortalities which explain the downsizing.

I think the answer is E. Is not it?

hmm.. I would go with E as well.

actually, (a) is the worst answer choice, because it weakens the argument.

the conclusion says:
The seasonal variation in group size can probably be explained by a seasonal variation in mortality among young voles.

this is crazy-talk for
"the groups are smaller in spring and summer because lots of young voles die, for some reason, in the spring and summer."

if (a) is true, though, then there is a higher % of young voles during the spring and summer, a statistic that's in direct contradiction to the thesis of the passage. not good.

--

(e) strengthens the correlation by not only providing a reason for the differential in sizes of vole families, but also showing specifically that it's the young voles that are killed (as required by the conclusion).

_________________
Being well-dressed gives a feeling of inward tranquillity [that] religion is powerless to bestow.
C.F. Forbes

Hi Ron,

Is D a weakener in this case?

i wouldn't call it a weakener. it rules out one possible cause of seasonal mortality -- i.e., the fact that the voles survive through the winter indicates that the harsh winters are not the cause of their mortality -- but there are plenty of other possible seasonal causes.

perhaps you thought that “seasonal” refers specifically to climate, in which case it would be understandable that you considered choice (d) a weakener.
however, this is not the case -- “seasonal” is an adjective that describes anything at all that changes according to the seasons.

_________________
Being well-dressed gives a feeling of inward tranquillity [that] religion is powerless to bestow.
C.F. Forbes

by elimination we can choose E

but I think E is incorrect. the strengthening action in E is not certainly happen. we have no action of snake's eating of voles and of course we do not have that this eating increases the belief that there is a causal relation.

if E said: in snakes eat many young voles in the time from Spring to Autum, E is correct because if this action happen we can not have certain conclution that there is a causal relation but we can increase the belief in the conclusion- the condition for a strengthener.

the strengthening or weakening actions must happen actually though they only increase belief or doubt in the conclusion (by increasing the belief or doubt in an assmption-classical Kaplan method)

Some question in Manhantan make us choose between 2 choices, strengthening action happens actually and strengthening action have not happen. I do not remember but will find and post here.
gmatprep quesions are hard beautifully but this one dose not make me happy.

I do not want to discuss this thing, honestly. But gmat score is so important that Havard business school would not look at our application carefully if we do not get 750

first --
OFFICIALLY CORRECT ANSWERS ARE CORRECT!
do not question officially correct answers!
far too many students on this forum make the mistake of questioning the correct answers; please note that doing so is a complete waste of your time and effort. i.e., exactly 0% of the time that you spend posting "isn't this official answer wrong?" is productive, and exactly 100% of that time is wasted.

"is this correct?" is NEVER a productive question to ask about one of GMAC's correct answers -- the answer is always yes.
"is this wrong?" / "is this X type of error?" is NEVER a productive question to ask about one of GMAC's correct answers -- the answer is always no.

instead, the questions you should be asking about correct official answers, if you don't understand them, are:
"why is this correct?"
"how does this work?"
"what understanding am i lacking that i need to understand this choice?"

this is a small, but hugely significant, change to your way of thinking -- you will suddenly find it much easier to understand the format, style, and conventions of the official problems if you dispose of the idea that they might be wrong.



this is actually the whole point of strengthening and weakening problems -- i.e., the entire purpose of these problems is to make sure that you can understand the most likely or most probable interpretation/outcome of what you are given

this is meant to mimic the actual reasoning of business itself -- in which almost nothing will ever be "certain to happen".



actually, we do -- the answer choice says that snakes are "a major predator of young prarie voles". that's what "predator" means -- they kill and eat their prey.



they are not going to spell this out for you explicitly. see above -- that's the whole point of this problem type: for you to see which factors make a causal relation more or less likely.

you should think of this kind of thing in the same way you'd think of, say, evidence presented in court.
very few pieces of evidence in a courtroom are going to prove the guilt or innocence of a defendant -- but many pieces of evidence will strengthen or weaken the case against the defendant.




this is, in essence, exactly what choice (e) says.



i assume you are just trying to use hyperbole here. but, just in case you are actually being serious, you should go look at harvard's numbers for a reality check.
the median score for admitted students at harvard is not even this high, so this statement is obviously wrong.

_________________
Being well-dressed gives a feeling of inward tranquillity [that] religion is powerless to bestow.
C.F. Forbes

Hi Ron,

How about (C)? I understand the choice is wrong but my own reasoning isn't crystal clear.

(C) states that the voles (young and adult) subsists primarily on broad-leaved plants abundant in spring. Despite there being no shortage of food the voles should be living in large numbers. Hence eliminated the option C

Is this the right way to reason?

IMO yes, I used the same reasoning to eliminate option C.

HTH.

manoj, i'll thank you not to keep responding to posts that start with "hi ron, ..."

--

samaresh --

that's basically the idea. if the animal's food supplies follow that sort of seasonal pattern, then the groups should be larger starting in the spring when there's a ton of food everywhere.

_________________
Being well-dressed gives a feeling of inward tranquillity [that] religion is powerless to bestow.
C.F. Forbes