Feathers recently taken from seabirds stuffed and

Question

Feathers recently taken from seabirds stuffed and preserved in the 1880s have been found to contain only half as much mercury as feathers recently taken from living birds of the same species. Since mercury that accumulates in a seabird’s feathers as the feathers grow is derived from fish eaten by the bird, these results indicate that mercury levels in saltwater fish are higher now than they were 100 years ago.

The argument depends on assuming that

(A) the proportion of a seabird’s diet consisting of fish was not as high, on average, in the 1880s as it is today

(B) the amount of mercury in a saltwater fish depends on the amount of pollution in the ocean habitat of the fish

(C) mercury derived from fish is essential for the normal growth of a seabird’s feathers

(D) the stuffed seabirds whose feathers were tested for mercury were not fully grown

(E) the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds’ feathers

Source: LSAT

VeritasPrepBrandon · Accepted Answer

Veritas teaches a technique called the "assumption negation technique" that is very effective for solving critical reasoning assumption questions. In order to utilize the assumption negation technique, narrow down your list of potential answers to a couple you feel may be correct. Then, for each answer choice, negate the choice, insert the negated statement back into the argument, and gauge whether or not the argument falls apart. A correct assumption, when negated, will destroy the argument.

This argument is stating that mercury levels are higher in saltwater fish now than in the 1880s, and it uses the comparison of the mercury in feathers of currently living birds vs. those of birds stuffed in the 1880s.

Negating answer choice E, we have: "the process used to preserve birds in the 1880s DID substantially decrease the amount of mercury in the birds’ feathers." If that is the case, then the comparison between living birds and those stuffed in the 1880s is no longer relevant, as the mercury in those birds' feathers was significantly altered. This destroys the link between premises and conclusion, and thus answer choice E is a required assumption.

I hope this helps!!!

testcracker · Answer

hi

its fairly easy problem, and like everyone, I also got it right, but...
I was wondering if you could tell me whether the way I have eliminated "C" is okay

Assumption is an unstated premise, but the premise that "C" states is clearly stated in the argument, so it cannot be an unstated premise, so I have ruled it out. Is that okay ..?

thanks in advance, man

nawaf52 · Answer

https://www.youtube.com/watch?v=5LdXSJwF2NA

Turkish · Answer

Went with E

(E) the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds’ feather.

The argument has to consider the answer E before making the statement "mercury levels in saltwater fish are higher now than they were 100 years ago"

Skywalker18 · Answer

Hi gmatcracker2018,
I don't think C is stated in the argument.

(C) mercury derived from fish is essential for the normal growth of a seabird’s feathers - Incorrect

mercury that accumulates in a seabird’s feathers as the feathers grow is derived from fish eaten by the bird -- We can't infer that mercury is essential for the normal growth based on this sentence.

adkikani · Answer

GMATNinja   nightblade354   VeritasPrepKarishma   pikolo2510   generis

Can you help me to understand the argument?

Birds from today have 100 mg of mercury (for e.g) than those that lived in 1980 who had 
50 mg of mercury. Both birds are of same species.

Mercury found in birds' feathers is derived from fish that birds eat.
There is a direct linear relationship between accumulation of mercury in feathers and 
amount of fish that birds eat.

Final conclusion:
mercury levels in  saltwater fish  are higher now than they were 100 years ago.

Here is where I faltered? What a scope shift from birds to fish in the argument? I am nowwhere given 
a hint to relate mercury level in fish. It is the birds that eat fish, how do I know mercury levels in fish and to
what do I co-relate it to?

Skywalker18 · Answer

Hi  adkikani ,

1. Feathers recently taken from seabirds stuffed and preserved in the 1880s have been found to contain only half as much mercury as feathers recently taken from living birds of the same species.

2. Since mercury that accumulates in a seabird’s feathers as the feathers grow is derived from fish eaten by the bird-->  the fish is the source of Mercury in the seabird's feather

these results indicate that mercury levels in saltwater fish are higher now than they were 100 years ago.---> Based on the premises 1 and 2 we can infer the given conclusion.

1.the proportion of a seabird’s diet consisting of fish was  not significantly lower , on average, in the 1880s as it is today.
2. The seabirds whose feathers have been stuffed and preserved since the 1880s  were not significantly younger  than those whose feathers were taken recently.

Are these also valid assumptions?

AjiteshArun  ,  GMATNinja  ,  mikemcgarry  ,  egmat  ,  RonPurewal  ,  DmitryFarber  ,  MagooshExpert  ,  ccooley  ,  GMATNinjaTwo  , 
  VeritasPrepKarishma  ,other experts-- please enlighten

pikolo2510 · Answer

the analysis of the argument was perfect

As you see there is a big scope shift. The assumption that should straight popup in your head is "Mercury in saltwater fish is directly responsible for the mercury in the bird's feather" . That's how you can draw the conclusion. Hope this helps!

nightblade354 · Answer

adkikani , the above explanations are good. As for me, I simply negated (E):

(E) the process used to preserve birds in the 1880s  did   substantially decrease the amount of mercury in the birds’ feathers 
 
If this is the statement, then the argument completely falls apart, and hence is the answer. Negation is always great on assumptions questions!

GMATNinja · Answer

Skywalker18 , you nailed it! We are specifically told that the "mercury that accumulates in a seabird’s feathers as the feathers grow is derived from fish eaten by the bird ." Although we aren't given the exact mercury levels in the fish, we can infer that, all else equal, increasing the amount of fish eaten would increase mercury levels in the feathers. Also, increasing the mercury levels in the fish would likely increase mercury levels in the feathers. And, yes, your highlighted assumptions (variations of choice A and choice D) would be valid assumptions. If either of those highlighted assumptions were not true, the argument would fall apart. You deserve a cookie.

Skywalker18 · Answer

Thank you GMATNinja sir for your kind words

warrior1991 · Answer

(E) the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds’ feathers

Negate this option:-  
 The process used to preserve birds in the 1880s DID SUBSTANTIALLY decrease the amount of mercury in the birds’ feathers.
This will make the argument fall apart. 
If the mercury levels were altered deliberately, the comparison between birds of 1880s and birds of today becomes absurd.

cataakash · Answer

@

But don't you think that when you negate B that also makes sense?
The amount of mercury in a saltwater fish does not depend on the the amount of pollution in the ocean habitat of the fish.

Please help

aditibhandari · Answer

Hi,

Here you are assuming that pollution is changed from the time period of 1880 to 1980, nothing sort of that information is provided in the passage.

gmat1393 · Answer

Hey  nightblade354

Please add the LSAT tag

Thanks

honey1 · Answer

Feathers recently taken from seabirds stuffed and preserved in the 1880s have been found to contain only half as much mercury as feathers recently taken from living birds of the same species. Since mercury that accumulates in a seabird???s feathers as the feathers grow is derived from fish eaten by the bird, these results indicate that mercury levels in saltwater fish are higher now than they were 100 years ago.

The argument depends on assuming that

(A) the proportion of a seabird???s diet consisting of fish was not as high, on average, in the 1880s as it is today

(B) the amount of mercury in a saltwater fish depends on the amount of pollution in the ocean habitat of the fish

(C) mercury derived from fish is essential for the normal growth of a seabird???s feathers

(D) the stuffed seabirds whose feathers were tested for mercury were not fully grown

(E) the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds??? feathers
I have a doubt why option A is not the answer and how does it weakens the argument and affects the main conclusion. how should we with such type of options.
THANK YOU

Prihans · Answer

Hi,
please explain why A is wrong. If the proportion of diet is sholuld be same then only it is possible that the fishes in the comparision may have same mercury. IF we consider E, it is saying preservation decreased the mercury level substantially but it is not clear that whether the preservation has decreased it from same level.(The level found in feathers recently).
Please clear my confusion

thanks

CrackverbalGMAT · Answer

Pre-thinking:

Conclusion:  Mercury levels in saltwater fish are higher now than they were 100 years ago.

Premises on which it is based: 
i) Mercury that accumulates in a seabird’s feathers as the feathers grow is derived from fish eaten by the bird.
ii) Feathers recently taken from seabirds stuffed and preserved in the 1880s have been found to contain only half as much mercury as feathers recently taken from living birds of the same species.

Prima facie the argument seems to make sense. If mercury in the feathers derives from fish eaten, and recent feathers have more mercury, it is logical to suggest that fish now have more mercury. Therefore, in order to find an assumption, we are looking for one of the following:

1) Rule out an alternate explanation.
2) Rule out a correlation (a third variable that influences both time and mercury in feathers).

Let us examine the answers:

(A) the proportion of a seabird’s diet consisting of fish was not as high, on average, in the 1880s as it is today  This is worth considering. If mercury comes from fish, and the seabirds are eating more fish today, it is possible that this has led to more mercury. However, this assumes that the other portion of the diet does not contain any mercury. For now, we can hold on to this option.

(B) the amount of mercury in a saltwater fish depends on the amount of pollution in the ocean habitat of the fish  This does not address the mercury in the feathers at all. Irrelevant. Eliminate.

(C) mercury derived from fish is essential for the normal growth of a seabird’s feathers  This does not address the difference in the mercury levels in the feathers at all. Eliminate.

(D) the stuffed seabirds whose feathers were tested for mercury were not fully grown  We know nothing about levels of mercury in the feathers of juvenile and adult seabirds. Out of scope. Eliminate.

(E) the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds’ feathers  Correct answer. This clearly rules out an alternate explanation for the different levels of mercury.

Since (E) is a much cleaner option, we can eliminate option (A).

Hope this helps.

Akshaynandurkar · Answer

Why A is wrong. 
Option A is strengtener and not the assumption
Definition of assumption : The minimum necessary condition required by conclusion. This means you don't have to go above the minimum requirement when it comes to assumption.
In case of strengthener , you can go above the minimum requirement. So basically, assumption is the subset of strengtheners.
Now coming to A. Why option A is strengthener ? Because it gives more than what is minimum required. for conclusion to hold true we need the minimum condition that proportion of seabird's diet in 1880s is same as it is today. 
option A says " not as high" which means it was lower in 1880s but do we really need it to be lower. (yes if it is lower, it strengthens the conclusion) but this is not necessary condition. It is not minimum requirement. Even if it is same proportion, it would be sufficient.

Hope this helps!

Feathers recently taken from seabirds stuffed and

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