Exposure to certain chemicals commonly used in elementary schools as

10 years back:
Total Number of schoolchildren sent to Elementary school nurses = 100
Number of schoolchildren sent to Elementary school nurses for treatment of allergic reactions to those chemicals = 10

So, proportion of schoolchildren sent to Elementary school nurses for treatment of allergic reactions to those chemicals = 1 in 10.

Now:
Total Number of schoolchildren sent to Elementary school nurses = 100
Number of schoolchildren sent to Elementary school nurses for treatment of allergic reactions to those chemicals = 20

So, proportion of schoolchildren sent to Elementary school nurses for treatment of allergic reactions to those chemicals = 1 in 5.

This is meant by increase in proportion of schoolchildren sent to Elementary school nurses for treatment of allergic reactions.

Since the argument are talking about "proportion" (percentage), the absolute "number" does not matter.

In this argument the premise is: the proportion of schoolchildren sent to school nurses for treatment of allergic reactions to those chemicals has increased significantly over the past ten years.

For the above premise, the argument says that the possible explanations are either of the following:

(X) Renston's schoolchildren have been exposed to greater quantities of the chemicals, or
(Y) they are more sensitive to them than schoolchildren were ten years ago.

But then, for the above premise, a third possible explanation can be:

(Z) Children who have allergic reactions to the chemicals are more likely to be sent to a school nurse now than they were ten years ago (the way in my previous example, teachers were more lenient in marking now than was the case 10 years ago).

However, since the argument is concluding that (X) or (Y) are the possible reasons, the assumption is that (Z) is not the reason (the way in my previous example, the assumption was teachers were not more lenient in marking now than was the case 10 years ago).

This is exactly what C states.

Observation: More and more children are being sent to the school nurses because of allergic reactions due to cleaning chemicals over the past ten years.

Conclusion: Either more chemicals are getting used or children are becoming more sensitive.

We are concluding that the cause of the observation is one of these two only. When we say that one of these two is the cause, we are assuming that nothing else is the cause of the observation (that more children are being sent to nurses for allergy).
So we are assuming that nothing else is the cause.

But there is another cause one can think of - what if the teachers are the ones getting more sensitive i.e. what if they are observing children more carefully? What if they are sending children to the nurses upon observing a tiny amount of allergy? What if because of this the children who get an allergic reaction today are more likely to be sent to the nurses than were the children 10 years ago. What if 10 years ago, the teachers did not bother that much until and unless they saw a case of full blown allergy?
That could be another reason why more children are being sent to the nurses now.

Note that the argument doesn't say that more children are getting allergic reactions. It very carefully says that more children are getting sent to the nurses (sneaky GMAT way)

(C) Children who have allergic reactions to the chemicals are not more likely to be sent to a school nurse now than they were ten years ago.

This is an assumption as we discussed above. For our conclusion to hold that only one of those two causes is possible, we are assuming that children who get allergic reactions are not more likely to be sent today than they were 10 years ago.

Answer (C)

Check out this post on assumptions: https://anaprep.com/critical-reasoning- ... ssumption/

As you suggest, if the number of elementary school children got bigger, and the rate of exposure to chemicals stayed the same, you'd probably expect more children to be sent to the nurse for treatment.

Notice, however, that (E) doesn't claim that the number of elementary school children stayed the same. Rather, it tells us they do not make up a larger proportion than they did ten years ago. This really isn't necessary to assume. Notice that the number of elementary school children could stay the same even if the proportion changed (depending on the number of non-elementary school people in the town). For this reason, (E) doesn't really relate to the conclusion.

From another angle: notice that the evidence supporting the conclusion doesn't claim that the NUMBER of school children sent to the nurse for treatment increased. Rather, it claims that the "proportion" increased. So even IF the number of elementary school children increased, this wouldn't explain why the proportion being sent to the nurse for treatment increased.

So we wouldn't really need to assume that the number of elementary school children stayed the same (though again, answer choice (E) is talking about the proportion of the town that are elementary school children, not the total number of elementary school children).

Since (E) isn't required to draw the conclusion, it isn't an assumption on which the argument depends, which makes it incorrect.

I hope that helps!

­Hi Gmatguy007,

Perhaps I can chip in here. 

There are a couple of points to discuss.

I am afraid you misunderstood what the given proportion is when you solved this one!

Let's deep dive into the pertinent sentence.

"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."

What the proportion is talking about -> Out of the schoolchildren sent to the nurses overall, how many were sent to the nurses for treatment of allergic reactions to those chemicals.

This is ( Number of children sent to nurses for treatment of allergies related to those chemicals / Number of children sent to nurses for treatment overall (including other reasons like stomach ache) )

This is not ( Number of students with allergic reactions / Number of nurses ). Number of nurses is nowhere in the picture here!

A quick example of proportion: If I say that the proportion of children sent to the principal's office for misbehavior increased, it means -> 

(No of children sent to the principal's office for misbehavior / Total number of children (Sent + not sent for the above reason) )

Bottom Line: The argument is not concerned with the proportion you have used - ( Number of students with allergic reactions / Number of nurses ). So, the logic you have used where you are trying to see how this proportion is impacted as a result of choice A - is incorrect.

I also wanted to focus on the bolded portion above as I feel you may be making an error here in interpreting what is given. 

Go through the argument once more - is it really given that the number of students sent to hospitals (for allergy treatment related to those chemicals) increased? 

We are only given that the proportion of such students increased. 

This is an important concept: An increase in proportion does not necessarily imply an increase in the absolute number.

For example:

10 years ago -> 100 students sent to hospital. 10 for allergies related to these chemicals. Proportion - 10%
Now -> 50 students sent to hospital. Again, 10 for allergies related to these chemicals. Proportion - 20%

The proportion can very well increase without an actual increase in the number. The argument only tells us that a proportion increased. We should be careful not to assume that the number increased too.

Hope this helps!

Cheers,
___
Harsha
Enthu about all things GMAT | Exploring the GMAT space | My website: gmatanchor.com

Summary - Nurses assume children are more exposed to chemicals, or more sensitive than children were 10 years ago as the # of cases has increased significantly.
Assumption - The likelihood of children sent to the nurse after being exposed is the same as it was 10 years ago. In other words, nothing has changed.




The number of nurses has decreased over the past ten years. Great, but it has no impact on the conclusion. OUT


I won't even try an opposite test on this option, as it's irrelevant. OUT!



Children who have allergic reactions to the chemicals are MORE LIKELY to be sent to a school nurse than they were ten years ago. If that's the case, it completely destroys the argument's conclusion that students are either more sensitive now, or are more exposed to the chemicals.

KEEP!



This option is similar to in other countries blah blah blah happens. We are only concerned with the use of the pesticides and cleaners at the school.
TO make this assumption work, we would also have to assume that even though the students were exposed at home, they were sent to the school nurse instead of going to the doctor at a hospital etc.

OUT!



This is irrelevant. OUT!


C is the answer IMO.

The argument concludes by stating that the increase in proportion of school children sent to elementary school nurses for treatment of allergic reactions to chemicals might be either because the students have been exposed to greater quantities of the chemicals, or that they are more sensitive to chemicals than schoolchildren were ten years ago. The argument is hence, assuming that these are the only 2 possible reasons for the increased proportion. Thus, we need to find an answer choice that eliminates any alternative reasons. What would cause the conclusion to break down ? - Pointing out an alternative reason. Possible alternatives : 1) What if the number of students has increased over the past ten years ? Then the proportion could be higher without children being more sensitive or being exposed to these chemicals on a greater scale. 2) What if earlier also the magnitude of the reaction was similar but children were not sent to schools then ( ten years ago) probably because it was felt that a treatment is not required, however over the period of ten years more children are being sent to nurses.

(A) The number of school nurses employed by Renston's elementary schools has not decreased over the past ten years. - The number of nurses is irrelevant; the argument is talking about the proportion of reported cases here.
(B) Children who are allergic to the chemicals are no more likely than other children to have allergies to other substances. - argument talks about allergies to only chemicals and not other substances.
(C) Children who have allergic reactions to the chemicals are not more likely to be sent to a school nurse now than they were ten years ago. - Yes, negate this: Children who have allergic reactions to the chemicals are more likely to be sent to a school nurse now than they were ten years ago. This would naturally lead in an increased proportion of students being sent to nurses and this breaks down the conclusion that there can only be two possible reasons.
(D) The chemicals are not commonly used as cleaners or pesticides in houses and apartment buildings in Renston. - This is irrelevant.
(E) Children attending elementary school do not make up a larger proportion of Renston's population now than they did ten years ago. - The comparison and argument is limited to elementary school children and is nowhere related to the rest of the population.

C is correct.

Summary - Nurses assume children are more exposed to chemicals, or more sensitive than children were 10 years ago as the # of cases has increased significantly.
Assumption - The likelihood of children sent to the nurse after being exposed is the same as it was 10 years ago. In other words, nothing has changed.

How is this the assumption. The arguments states lementary school nurses in Renston report that the proportion of schoolchildren sent to them for treatment of allergic reactions to those chemicals has increased significantlyover the past ten years. It says proportion of school children sent for treatment has increases significantly. Can you please explain?

sayantanc2k GMATNinja,

I understand option A is clarified above and why it is wrong but I am not convinced yet. Proportion has increased that can also mean that nurses are same in count and students count has increased and so they are more cases as compared to past.

This 'NOT' in option C is confusing me.

Posted from my mobile device

I'm glad that i chose the right answer (c) but i really struggled a lot and it took me a lot of time.

I noticed that i struggle with negative modifiers with degrees such as "not more likely". When i really thought about this phrase it just means it is NEUTRAL. So when I substituted "not more likely" with the idea of it being just neutral. I understood it more.

As a non native english speaker, i struggle with tricky negative words like the one above.

Does anyone have tips with tricky words like this?

Posted from my mobile device

But in A, the number of nurses matters? Because if the nurses number decrease, the proportion will be increase though after total number of students will not increase. So,shouldn't we assume that the number of nurse hasn’t decreased ? How A is wrong?

Posted from my mobile device

I'll steal a bit from this post that explains a similar issue with answer choice (E):

The author is trying to explain the proportion of elementary school students sent to nurses for treatment of allergic reactions:

• This proportion is a % of elementary school students.
• In a fraction, it would be expressed as [# of elementary students sent to the nurse for treatment of specific allergic reactions] / [total # of elementary students].

You can see that the number of nurses plays no role in the relevant proportion -- we only care about the number of students sent to the nurse for treatment as a proportion of the total number of students.

I hope that helps!

Passage Analysis

• Exposure to certain chemicals commonly used in elementary schools as cleaners or pesticides causes allergic reactions in some children.

o Exposure to some particular chemical substances causes allergic reactions in some children.
o These chemicals are commonly used in elementary schools as cleaners or pesticides.

• 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.

o Elementary school nurses in Renston have reported about the proportion of school children’s allergic reactions.
o The nurses state that over the last 10 years, the proportion of schoolchildren sent to treat allergic reactions to the abovesaid chemicals (as a fraction to the total population of school children) has risen by a significant number through the period.

• Therefore, either Renston's schoolchildren have been exposed to greater quantities of the chemicals, or they are more sensitive to them than schoolchildren were ten years ago.

o The author concludes that this phenomenon occurred because of either of the two possible reasons listed.

 One scenario is that the school children at Renston are exposed to higher amounts of the allergy causing chemicals.
 Another possibility is that the school children of Renston today are more likely to respond with allergic reaction to the same quantity of chemical exposure as the school children ten years ago were subjected to.

Conclusion: Either Renston's schoolchildren have been exposed to greater quantities of the chemicals, or they are more sensitive to them than schoolchildren were ten years ago.

Question Stem Analysis
We are required to identify the assumption needed to reach the conclusion of this argument.

Pre-thinking

Falsification Question
In what scenario is it possible that neither Renston's schoolchildren have been exposed to greater quantities of the chemicals, nor they are more sensitive to them than schoolchildren were ten years ago?
Given that

• Exposure to certain chemicals commonly used in elementary schools as cleaners or pesticides 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.

Thought Process
The two possibilities listed by the author as reasons for the observed increase in the proportion of school children sent to be treated for allergies to the said chemicals are, either that the children today are exposed to more quantity of the chemicals, or that the children these days are more sensitive to the chemicals and prone to have allergic reactions triggered by lesser amounts of the same. The implicit assumption made by the author lies in the link between the actual number of school children who had allergic reactions and the number of schoolchildren sent to the nurses for this reason. An assumption has been made regarding the strength of the chemicals in question as well.

Falsification Condition#1
What if students were not sent to nurses for allergic reactions to the chemicals ten years ago as often as they are sent for the same reason now? In that case the facts hold but the conclusion breaks down.
Assumption#1
Children who have allergic reactions to the chemicals are not more likely to be sent to a school nurse now than they were ten years ago.

Falsification Condition #2
What if, during the last ten years, the strength of the chemical used in the cleaners and pesticides has increased? In that case, neither of the two possibilities listed in the conclusion will be needed and the conclusion will break down.
Assumption#2
The strength of the chemical used in cleaners and pesticides has not increased during the last ten years.

Answer Choice Analysis

(A) The number of school nurses employed by Renston's elementary schools has not decreased over the past ten years.
INCORRECT
This option may confuse if the word ‘proportional’ in the passage is misunderstood as representing the ratio of schoolchildren sent for treatment to the number of nurses. But ‘proportional’ actually compares the allergic schoolchildren sent to be treated to the total population of school children. In this case, the number of school nurses makes no difference to the conclusion. Therefore, it is an incorrect answer.

(B) Children who are allergic to the chemicals are no more likely than other children to have allergies to other substances.
INCORRECT
Allergies to other substances are totally out of context to our argument. Hence, this choice is an incorrect answer.

(C) Children who have allergic reactions to the chemicals are not more likely to be sent to a school nurse now than they were ten years ago.
CORRECT
This information is necessary for the argument to hold and it is in line with our pre-thinking. Hence this is the correct answer choice.

(D) The chemicals are not commonly used as cleaners or pesticides in houses and apartment buildings in Renston.
INCORRECT
The usage of the chemicals in question anywhere outside elementary schools does not come under the purview of the argument, and hence, this option forms an incorrect choice.

(E) Children attending elementary school do not make up a larger proportion of Renston's population now than they did ten years ago.
INCORRECT
We are concerned only about the proportion of elementary school children sent to be treated for allergic reactions due to the mentioned chemicals as a ratio to the total number of elementary school children. The fraction the latter constitute among the total population of Renston is not a relevant factor in our analysis. Hence this is also a wrong answer choice.

The correct answer is option (C).

Understanding the passage:

1. Exposure to certain chemicals has caused allergies in school children
2. These chemicals are used as cleaners or pesticides in the elementary schools
3. In Renston, elementary school nurses say that - "the proportion of children sent to them for allergy treatment has increased significantly over the last 10 years"
4. Conclusion: Either 1) Renston's schoolchildren have been exposed to greater quantities of the chemicals (OR)
2) They are more sensitive to chemicals today than students from 10 years ago

Understanding the question
Find the assumption on which the argument depends.

Thought Process:

An assumption is that implicit thought which is not stated, but has to be true for the conclusion to hold true. So, if an assumption is not true (negated), then the conclusion should break. A failsafe way to identify the assumptions is to identify the conditions under which the conclusion would definitely break.

So, let us falsify or break the conclusion:

In what scenario - 1) (Neither) have Renston's schoolchildern been exposed to greater quantities of the chemicals
2) (Nor) are they more sensitive to chemicals today than students from 10 years ago

Given that 1) Exposure to these chemicals cause allergic reactions in school children 2) The proportion of students reporting to nurses for treatment of allergies due to these chemicals has increased.

Falsification Condition 1:
What if the concentration of the harmful component in these chemicals has increased, from ten years ago?
In such a case, even if the exposure level is the same as ten years ago (i.e. the amount of chemicals the children are exposed to is same), the amount of the harmful component of the chemical the children are exposed to will be more.

Example: 10 years ago, 50 grams of harmful component in 100 grams of chemical. Now, 75 grams of harmful component in every 100 grams of chemical. In terms of exposure to the chemical, the amount is still the same (say, 100 grams), but the exposure to the harmful component is higher (because of higher concentration (75 grams today instead of 50 grams 10 years ago)

Assumption 1: The concentration of the harmful component(s) in the chemicals has not increased today, as compared to 10 years ago

Falsification Condition 2:
What if students 10 years ago did not visit the nurse for allergies as often as students these days?
Maybe a significant number of students never visited the nurses for allergies, for whatever reason (maybe medicine those days was not good enough to justify going to a medic!). Then the increase in proportion observed is not because of the reasons mentioned in the conclusion, but because of this increased likelihood of visiting a nurse.

Assumption 2: Children who develop allergic reactions to the chemicals are not more likely to visit a nurse today, than they were 10 years ago

Let us look at the answer choices:

(A) The number of school nurses employed by Renston's elementary schools has not decreased over the past ten years.

Irrelevant. The conclusion is about schoolchildren visiting the nurses for allergies due to chemicals. The number of nurses has no relation to the conclusion.

(B) Children who are allergic to the chemicals are no more likely than other children to have allergies to other substances.

Irrelevant. The passage is specific to the chemicals used in cleaners/pesticides, which can cause allergies in school children. Any other substance has no impact on our conclusion, and cannot therefore, be the correct assumption.

(C) Children who have allergic reactions to the chemicals are not more likely to be sent to a school nurse now than they were ten years ago.

This is in line with our 2nd assumption. If children today are more likely to visit a nurse for allergies from these chemicals, then the increased proportion can be because of this increased likelihood rather than the points mentioned in the conclusion. Hence, this is the correct choice.

(D) The chemicals are not commonly used as cleaners or pesticides in houses and apartment buildings in Renston.

Let us try negating this assumption. Even if these chemicals are commonly used as cleaners/pesticides in places other than elementary schools, that only means that the children may be exposed to even more quantities of the chemicals than previously assumed. This may be happening 10 years ago as well (if the same chemicals were used then too), but we do not know. In no way will this option break the conclusion.

(E) Children attending elementary school do not make up a larger proportion of Renston's population now than they did ten years ago.

Irrelevant. The only relevant proportion in this argument is the proportion of students who went to nurses. The proportion of children in the population has zero impact on the conclusion.

Cheers!

(A) The number of school nurses employed by Renston's elementary schools has not decreased over the past ten years.

Doesn't that make much sense that a reduced number of nurses mean that each nurse will attend more children?
Suppose the number of nurses has been decreased to half, then each nurse will attend twice the number of children, thus they ended up reporting the "higher proportion" of schoolchildren being sent to them for the treatment.

Can you please tell me why my justification is flawed?

Take a look at this post that explains the reasons for eliminating (A).

Your reasoning isn't flawed, but you're looking at the wrong thing. The argument is about the proportion of students sent to a nurse -- in other words, what fraction of the student population is sent to a nurse. The number of nurses or the ratio of nurses to students is irrelevant.

I hope that helps!