PT #55 S. 3. Q 14

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cloudhidden
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PT #55 S. 3. Q 14

Postby cloudhidden » Thu Sep 20, 2012 1:56 pm

I'mhaving a hard time understanding this question. With answer choice (B) we can infer that if a building were not a dwelling then it's probably not made of limestone only. But how can we infer anything from knowing that a building is not made only from limestone. Say we have 100 buildings, 90 are dwellings and 10 are not dwellings. 75 of these buildings were made only of limestone and 25 were made from more than limestone. Answer (B) could have us distributing the limestone only buildings so that all 75 were found in human dwellings. That leaves us with 25 buildings made of more than limestone to distribute. We could assign all 10 of the buildings that weren't dwellings to buildings made up of more than limestone but that still leaves us with 15 more buildings made up of more than limestone to distrubute to dwellings. So in this scenario, if we know that a building isn't a dwelling then it's obviously more likely that it's made up of more than limestone, but what we cannot infer given that we know a building is made up of more than limestone that it's more likely that it's not a dwelling because then it's still 50% more likely that it's a dwelling. This scenario would weaken the conclusion. Thus we must make an additional assumption about how we play with the numbers. I thought strengthen and weaken questions were not supposed to require additional assumptions! Please help with this confusion.

Nat Sherman
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Re: PT #55 S. 3. Q 14

Postby Nat Sherman » Thu Sep 20, 2012 6:45 pm

Lets go with your original example. We have 100 buildings, 90 dwellings and 10 non-dwellings. There are 75 buildings of only limestone and 25 buildings made from more than just limestone. The question says we found a building made of more than just limestone, what is our chance that it is a non-dwelling, which that's easy there's a 10% chance it's a non dwelling. Okay, now lets try to strengthen that with one of the answers.

If we assume (B), that most of the non-dwellings were made from other stone. Since we know most means 51%-100%, lets use 50% since its the minimum. If 50% of the non dwellings are made of multiple materials, we know that 5 are made of multiple materials and 5 are not. So now what is the probability that the random building we found in the original premise is a non-dwelling.

We have 25 multi-stone buildings that we know 5 are non-dwellings. 5/25 = 20%. Since 20% is greater than 10%, (B) strengthens the conclusion that what they are studying is a non-dwelling

Basically in the original question we're given two things that we have dwellings and non-dwellings, and that we have buildings made out of limestone and those made out of other materials. Without the answer choice (B) there is no correlation between the type of stone and the type of dwelling it is, making the fact that they found a multi-stone building and concluding it is a non-dwelling was unwarranted. However, we can strengthen that conclusion by saying that most of the buildings at the site that were non-dwellings, were made of multiple stone.

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cloudhidden
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Re: PT #55 S. 3. Q 14

Postby cloudhidden » Thu Sep 20, 2012 8:17 pm

Thank you for the detailed and thought out response. I had been looking for an answer that would raise that probability above 50%, in other words, a sufficient assumption, and not the mere strengthener we needed to just boost that probability however much. I think the mathematical component to some LR questions can cause me to lose sight of the task in the question. But now I'm aware and prepared.




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