[CR Question] Please help answer

Question

Exposure to certain chemicals commonly used in elementary schools as cleaners causes allergic reactions in some children. Elementary school nurses in Renston report that the proportion of schoolchildren sent to them for treatment of allergic reactions to those chemicals has increased significantly over the past ten years. Therefore, either Renston's school children have been exposed to greater levels of the chemical, or they are more sensitive to them than schoolchildren were ten years ago.
Which of the following is an assumption on which the argument depends?
A. Children who have allergic reactions are not more likely to be sent to a school nurse now than they were ten years ago.
B. Children attending elementary school do not make up a larger proportion of Renston's population now than they did ten years ago.

Which is the correct answer? Please clearly justify why the second one is wrong. The other choices were irrelevant thus did not include.

KarishmaB · Answer

The first is the correct answer.

Nowdays, Renston's school nurses are receiving a higher proportion of children for treatment of allergic reactions. Say, if 10% were coming earlier, now 20% are coming. Note here that we are not bothered about the actual number of kids - whether it has increased or decreased - we are worried about whether the proportion has increased or decreased. The reason for the increase is given as one of the two:
- Children have been exposed to higher level of chemicals.
- Or children are more sensitive now.

Another thing that could explain the increased proportion (20%) is that now more children are sent to nurses than before. Say, if before also 20% kids experienced a reaction but the teachers did not bother to send them because it wasn't serious but now teachers send them no matter how minor the reaction - that could explain why 20% kids come to nurses now. 
This is option (A). When you say in the argument that only 2 possibilities are there, you are assuming that there is no third possibility. Option (A) says that a third possibility (children with allergic reactions are more likely to be sent to nurse) is not there. Hence it is an asusmption.

Option (B) - The proportion of children in the population has nothing to do with this. We are talking about the increase in the proportion of children sent to nurses. Even if now there are more children than before, all we need to see is what percent of them were sent to nurses. The proportion is what increases.

Gaurav2690 · Answer

Hi Karishma,
 Thanks for explaining this question. You have articulated it very well,and It is the best explanation that I have fount till now. I actually went through many posts related to this question but they were not so appealing as this. Thanks again.
 But I have one doubt about question stimulus "proportion" this word, what does this implies. I mean there two possibilities in my mind about this.
 1) it could be 
 Allergic children / Total number of children. 
 OR
 2) Allergic children due to chemicals / total number of allergic children.

These possibilities come to mind because if we you refer to actual question from OG15(CR 106), option (b) explicitly states these two cases and option (c) states one case.
 Though these cases wont affect the solution of problem, still if one became aware of actual scenario then he/ she could solve this with different perspective.

KarishmaB · Answer

Note what the question stem says "...the proportion of schoolchildren sent to them for treatment of allergic reactions to those chemicals"

The proportion of school children sent to them for allergic reactions to those chemicals...

This is the same as "the fraction of school children sent to them for allergic reactions to those chemicals..."

So this proportion is : Allergic children due to chemicals / Total number of school children

Gaurav2690 · Answer

Thanks for clarifying that. The confusion for me was between Idiom of ratio and proportion. I.e. 1)ratio of x to y. 2)proportion of x. So when I read proportion, I confused it with ratio and immediately started finding for "to" and I got "to those chemicals",which was illegal comparison. Thanks again.

DmitryFarberMPrep · Answer

A proportion *is* a ratio, but the ratio here is implied: it's the ratio of (children sent to the nurse for treatment of allergic reaction to chemicals) to (total children). It wouldn't make sense to have a ratio of children to chemicals. The GMAT would never use language like that.

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