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# The prevalence of a simian virus has been directly correlated with pop

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Manager
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21 Dec 2015, 04:53
Hi mikemcgarry

basically in this question we are testing two kind of gorillas to know they are infected from virus or not.

in other words its an sampling question. we are taking samples and then concluding .

here in option C and D both indicates sampling is not proper then why D is better then C?

according to option C if we take 100 gorillas of type M and 10 gorillas of type K then also sampling is biased.

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21 Dec 2015, 21:44
When i read the question it sounds like there is no paradox at all. MPG is more likely to be infected than KPG. So odds against the number of MPG is more than KPG. So is the survey. Population density of KPG > population density of MPG. Had the survey been the other way then there is a paradox. I would like to know what others think. Paradox should have conflicting ideas and here both are linear.
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Re: The prevalence of a simian virus has been directly correlated with pop  [#permalink]

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06 Feb 2016, 13:05
I went with C, since the results of the tests are biased.Note that the likelihood of the infection is pure speculation, as the evidence is based on the results of the tests.
Suppose we tested 100 MG and 10 KG.
from tested MG, 10 are infected
from tested 10 KG, 5 are infected.
But what if from the whole population of KG, the infected ones are much more? say 50%?
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07 Feb 2016, 09:07
pkm9995109794 wrote:
When i read the question it sounds like there is no paradox at all. MPG is more likely to be infected than KPG. So odds against the number of MPG is more than KPG. So is the survey. Population density of KPG > population density of MPG. Had the survey been the other way then there is a paradox. I would like to know what others think. Paradox should have conflicting ideas and here both are linear.

I think there is a paradox. It is really subtle
By 'More likely' they mean more than 50% .Lets say 70%. So 70% OF POPULATION OF MPG is infected AND MPG Population is less dense ..... (1)
Now that MPG is twice more likely than KPG , that means only 35% OF POPULATION OF KPG is infected AND KPG Population is highly dense .... (2)
FROM (1) & (2) => More Dense population has LESS Prevelence (LESS % of infected) ....... (3)
BUT
What The First line says ? Prevalence is directly proportional to density .
So THE MORE DENSE the population , THE MORE % of Infected
Is this we get from (3)? No just the opposite

D resolves it
D says ,
may be , because of aggressiveness of infected KPG they are less tested. And hence the resulting DATA is merely an approximation and hence does not reflect the practical Number
And
may be , because of calmness of infected MPG , they are well tested and hence the DATA is more accurate
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19 Dec 2017, 18:57
Premise: Simian virus is directly correlated with the population density in gorillas. That is, More the population density(no. of gorillas in a square kilometre or mile) more the simian virus. In layman terms , the more crowded the area the more is simian virus. SIMIAN VIRUS IS CONTAGIOUS. (This has to be kept in mind and is not given explicitly in the stimulus).
Premise: Republic of Dunaga has 2 areas Morgania Plain and Kogula Mountains. There are gorillas in these 2 areas... both are classified differently respective to their habitats.

Conclusion 1: Morgania Plain gorillas are more infected than Koluga gorillas.
Conclusion 2: The popualation density of Koluga gorillas is greater than Morgania gorillas.
What does that mean Koluga gorillas should me infected more than Morgania gorillas. But,we have a discrepancy in the results.

A: Kolugans leave mountains and wander in plains of Morgania for food... so the researchers might have mistaken the identity of Kolugans... this isnt possible once one does classification of gorillas. After classification researchers will not miss the identity. Wrong Answer.

B: Dormant strains of virus are often difficult to detect... so this assumes that Kolugans might have dormant strains of virus... which were not found in the testing. Could be the answer.. lets keep this.

C: More Kolugans and less Morganians... should yield more Kolugan virus infected gorillas and less Morganian virus infected gorillas.. since Kolugans have greater population density ... that is they live a lot closer to each other.. the greater the virus too.. from the stimulus. This choice contradicts the stimulus ... Wrong Answer.

D: Kolugans once infected anre more difficult to testing... could be the answer as... since these gorillas behaved agressively once infected...and hence were difficult to test.. there was lesser data available for Kolugans.. with respect to Morganians. Keep this one..

E:Both have similar markings but different subspecies... its fine... but classification takes into account many things and does not depend only on visual identity. So this is wrong.

Analysing our winners we have B and D.

B:Dormant strains are difficult to test... but can be tested with advanced techniques.
D: Aggressive gorillas are difficult to test... but can be tested if subdued.. which is again difficult to test.

Lets see the meaning of dormant.. it says something which is inactive.. here we have inactive virus.. and can be activated once the conditions are right.. that is still the gorillas are infected. Dormant or not dormant the virus is still present.

So finally we arrive at D. The answer.
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15 Jan 2018, 06:22
C makes the problem worse. It is because C should only explains the test results, but cannot explain why population density of Koluga gorillas is greater.
Hence, C is a great trap.
There is a similar question in which the answer is still D, but the trap is that regularly travels to other area.
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16 Jan 2018, 03:13
why option d is correct instead of option c
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16 Jan 2018, 18:47
rishabhmishra wrote:
why option d is correct instead of option c

The prevalence of a simian virus has been directly correlated with population density in gorillas. Recent fieldwork in the Republic of Dunaga, based on capturing gorillas and testing the gorillas for the virus, has shown that Morgania Plain gorillas are more than twice as likely to be infected than are the Kogula Mountain gorillas. Nevertheless, the population density of Koluga gorillas is significantly greater than that of Morgania gorillas.

Which of the following could best account for the discrepancy noted above?

A. During periods of little rainfall, Koluga gorillas often wander down into the plains in search of food.
B. Dormant strains of the simian virus are often difficult to detect.
C. Due to the Morgania gorilla’s natural habitat and its less reclusive nature, researchers have tested a greater number of Koluga gorillas than Morgania gorillas.
D. Infected Koluga gorillas behave very aggressively and are more difficult to subdue for testing.
E. The Koluga and the Morgania both have similar markings on their backs but are classified as different subspecies

IMO, you are confusing numbers with percent values.

Option C says - More Koluga were tested than Morgania. So we have more sample for Koluga but it doesn't explain the discrepancy in finding. We need some reason in sample size for the discrepancy. Now option D, It is hard to catch infected Koluga so the team is more successful in catching uninfected Koluga thus making our tested population bias.
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Re: The prevalence of a simian virus has been directly correlated with pop  [#permalink]

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16 May 2018, 19:26

MAGOOSH OFFICIAL SOLUTION:

The credited answer is (D). If this is true, then it may be that, as expected from the correlation, Koluga gorillas actually would be infected at a higher rate, but because the infected Koluga gorillas are aggressive and difficult to test, fewer of them actually would be tested, and most Koluga gorillas tested would wind up being the un-infected ones; this would lower the rate of ones that appear to be infected. This would explain the discrepancy.

According to the correlation model, more Koluga gorillas should be infected. If some move down to the plains for segments of the year, then during those times they might experience lower population densities, but then rain comes, and they go back up the mountain to rejoin the high density conditions, where they easily could be infected. This would not affect the overall infection rate. Choice (A) is incorrect.

If dormant strains of the virus are difficult to detect, presumably it would be difficult to detect in both Koluga gorillas and Morgania gorillas, and doesn’t explain a difference in rate between the two. Choice (B) is incorrect.

The argument is about percentages, about rates. We are trying to explain a discrepancy in expected rates. Actual numbers of one kind vs. the other kind of gorilla are irrelevant. Choice (C) is irrelevant and incorrect.

As long as researchers can tell the difference between the two kinds of gorillas, which doesn't seem to be an issue, then coloring similarities and species classifications are irrelevant. Choice (E) is irrelevant and incorrect.

Why can't Choice C explain the discrepancy in the data? Couldn't a larger sample of gorillas include more infected ones?

It's definitely true that the a sample size for a study like the one described in the question can skewer the results of that study. However, in this case, we'd actually expect the sample of Koluga gorillas to display abnormalities instead of the sample of Morgania gorillas, since the sample of Koluga gorillas would be smaller. After all, the larger a sample is, the more we can trust the data that it represents. So it looks like Choice C won't do a whole lot to explain the specific discrepancy that is mentioned in the question.

Choice D does a much better job of this because it describes an important behavioral difference between infected and non-infected Koluga gorillas. If infected Koluga gorillas act in a way that inhibits their sampling (being aggressive), that will have a strong impact on the results of researchers' studies, since the infected gorillas actually won't make it into the sample in the first place.

https://gmat.magoosh.com/questions/7219/a/212905261
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16 May 2018, 20:28
HoaTran wrote:
Here is how I analyze the issue. 100% sure that D is the answer.

1) population density is correlated with virus prevalence
2) But the proportion of KM gorillas tested infected is lower than that of MP gorillas and KM gorillas' density is higher than MP gorillas' density.

If 1 is true then it should have been instead
"the proportion of KM gorillas tested infected is lower than that of MP gorillas and KM gorillas' density is lower than MP gorillas' density"

The discrepancy can be explained by any reasons that show that the fieldwork result is skewed or inaccurate.
(D) clearly points that the sample of KM gorillas tested is skewed due to their aggressiveness and "unwillingness to co-operate with researchers" (just kidding, this is how I make GMAT fun)

Not sure how D is 100% correct here. It only mentions, aggressive gorillas are difficult to test. 'Difficult' does not exactly translate to 'not tested'.
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17 May 2018, 02:45
HoaTran wrote:
Here is how I analyze the issue. 100% sure that D is the answer.

1) population density is correlated with virus prevalence
2) But the proportion of KM gorillas tested infected is lower than that of MP gorillas and KM gorillas' density is higher than MP gorillas' density.

If 1 is true then it should have been instead
"the proportion of KM gorillas tested infected is lower than that of MP gorillas and KM gorillas' density is lower than MP gorillas' density"

The discrepancy can be explained by any reasons that show that the fieldwork result is skewed or inaccurate.
(D) clearly points that the sample of KM gorillas tested is skewed due to their aggressiveness and "unwillingness to co-operate with researchers" (just kidding, this is how I make GMAT fun)

Not sure how D is 100% correct here. It only mentions, aggressive gorillas are difficult to test. 'Difficult' does not exactly translate to 'not tested'.

I had the same exact thought. However, in comparison with the other answers D remains the best choice. Additionally, it seems likely that an agressive gorilla may not be tested at all if you picture it in your mind.
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25 Jan 2019, 23:39

I am equally split between C and D.
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26 Jan 2019, 03:22
mba1382 wrote:
The prevalence of a simian virus has been directly correlated with population density in gorillas. Recent fieldwork in the Republic of Dunaga, based on capturing gorillas and testing the gorillas for the virus, has shown that Morgania Plain gorillas are more than twice as likely to be infected than are the Kogula Mountain gorillas. Nevertheless, the population density of Koluga gorillas is significantly greater than that of Morgania gorillas.

Which of the following could best account for the discrepancy noted above?
A. During periods of little rainfall, Koluga gorillas often wander down into the plains in search of food.

B. Dormant strains of the simian virus are often difficult to detect.

C. Due to the Morgania gorilla’s natural habitat and its less reclusive nature, researchers have tested a greater number of Koluga gorillas than Morgania gorillas.

D. Infected Koluga gorillas behave very aggressively and are more difficult to subdue for testing.

E. The Koluga and the Morgania both have similar markings on their backs but are classified as different subspecies.

IMO Between C and D

C is actually going against the sample space, by this i mean that

eg If there were Overall 100 gorillas, 40 were MG and 60 were KG, so if initially 10 MG and 10 KG were infected,by the time they went to examine the infected, the infected MG got increased by more than twice, this increased the initial sample space(MG) of 10 to 30 for that matter.

So this doesn't address any discrepancy, its like they will keep on getting infected, does this give any reason for that, this is more like a statement

Now D says, Infected Koluga gorillas behave very aggressively and are more difficult to subdue for testing.
If Infected KG gorillas behave very aggressively they dont allow scientists to examine them, thereby explaining the discrepancy.
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Re: The prevalence of a simian virus has been directly correlated with pop  [#permalink]

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26 Jan 2019, 07:11
@arptikansal, the OA from magoosh has been posted, and there are bevy of posts above it to help eliminate (C). If you still cannot understand the question even after reading these, I can post an explanation.
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Re: The prevalence of a simian virus has been directly correlated with pop  [#permalink]

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27 Jan 2019, 09:54
Re: The prevalence of a simian virus has been directly correlated with pop   [#permalink] 27 Jan 2019, 09:54

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