#23 - Zoologist: Plants preferentially absorb heavy nitrogen

[Administrator]

Please post below with any questions!

[lll7]

This question really confused me. Can someone help me out?

I chose E on the test, but I wasn't really sure about it. Why is D right?

[Emily Haney-Caron]

Hi lll7,

Here's how the argument can be summarized:
Meat eating animals have even higher nitrogen levels than herbivores
Bone samples from ice age bears have the same nitrogen levels as blood samples from today's bears
Today's bears eat meat
Therefore, ice age bears ate some meat

This only follows if the readings from bone samples and blood samples are equivalent; otherwise, we might be comparing apples to oranges! That's what D is saying.

E, on the other hand, doesn't tell us a whole lot, because we're not comparing bone to bone, we're comparing bone to blood.

[jmramon]

Hello,

I can see why D is correct, but just want to be sure about understanding why B is wrong. Is B a "shell answer" because it tangentially addresses the concerns of D, but is ultimately irrelevant since the rate of accumulation isn't the stimulus' topic and rather accumulation generally is the focus? Thanks for your help:)

[Jonathan Evans]

JMRamon,

Good analysis. This argument itself includes a classic Shell Game™ between bone samples and blood sampled (really a primo example of Shell Gamin' if ever there were one). Answer Choice (B) is also Shell-ish—you've identified the issue; we don't care about "rate"—but sometimes these kinds of "new information" answers can be helpful in Strengthen and Weaken situations, depending on the argument. The issue here is twofold:

1. First, (D) without a doubt most strengthens the conclusion because it makes the connection explicit between "bone samples" and "blood samples." The information in Answer Choice (B) can't hold a candle to it.
2. Second, Answer Choice (B) actually shifts the focus to providing information about the rate of accumulation, as though we cared. The rate might be useful if it told us something about the integrity of the information about the overall levels, but this answer choice seems to be interested in rate qua rate. We certainly are not.

Thus, (B) is a two-time loser. (D) is way more on point, and (B) tries to take us down an irrelevant, primrose path to wrong answer-ville.

I hope this helps.

[relona]

Please post below with any questions!

Hi!

During my analysis after this practice exam, I prephrased the answer choice to say that regardless of how heavy plants were in the diet of the herbivore European cave bears, the heavy nitrogen in their bone samples still would not have equalled the levels of heavy nitrogen in the blood samples of present day bears. Although this prephrase did not lead to an answer choice, is this an accurate prephrase?
-Relona

[Adam Tyson]

That sounds like you were prephrasing a Weaken answer rather than a Strengthen answer, relona! But you have identified the problem in the argument: the author seems to think that a comparison of old bones to current blood samples is a valid one. Answer D supports that assumption by showing that blood samples are a good indication of what one would find in the bones, so it strengthens the argument by removing the problem that you were concerned about. Be sure that your prephrase accomplishes what the question asks you to do!

[CristinaCP]

I had a question about C. I think it could strengthen because it guards against the possibility that the levels of nitrogen in the samples only seemed identical because of different sample sizes. Maybe they had to collect bones from 20 different European cave bears to get the same amount of heavy nitrogen as blood samples from 3 modern bears. Then we wouldn't know if the amount of heavy nitrogen in an individual prehistoric bear would match the amount of heavy nitrogen in a modern bear.

But even if C does guard against that possibility, doesn't address the core issue of the argument: we don't know if we can compare prehistoric bone samples to modern blood samples to make a conclusion about whether European cave bears ate meat.

So is C wrong because even though it could guard against a potential weakening factor, it doesn't address the more glaring gap?

[Robert Carroll]

CristinaCP,

There's just no issue with having differently-sized samples. I can't see how it would matter at all. Consider that we do a poll of 1000 Americans and 1000 Europeans to try to get information about the respective attitudes of Americans and Europeans. How could it hurt that poll to instead pick 2000 Europeans? It can't. Note that it's not necessarily helpful, either - I'm concerned the samples aren't representative, but that's not a "sample size" issue.

Small sample sizes can be questionable because the inferences made from them may be reflective of "outliers" of the general population sampled. So a small sample is generally bad, a large one generally better. But...if a certain sample size is "big enough" to be reliable, surely there's no issue adding even more to the sample? Returning to my example, if 1000 Europeans is good enough, why not 2000? The number of Europeans is unequal to the number of Americans now, but if 1000 of each was fine, how does adding 2000 Europeans affect the validity of inferences about Americans?

What I'm trying to point out is that inequality of sample sizes between two groups isn't a weakness, and thus eliminating it won't strengthen an argument.

Robert Carroll

[H714W7]

Above, Jonathan Evans says "First, (D) without a doubt most strengthens the conclusion because it makes the connection explicit between "bone samples" and "blood samples."

I understand that B has a major problem because it is discussing the ratio, but does it not make an explicit connection between blood and bone samples? Did Johnathan mean that it doesn't explicitly connect the concentrations of nitrogen in blood samples to the concentration of nitrogen in bone samples?

Additionally, if B had been discussing blood levels the entire time, rather than blood ratios, would it also be problematic for B to say that concentrations can be *inferred * which suggests that you might have to do some kind of mathematical calculation to figure out what the blood amount would have been based on the bone amount? If so, it seems the passage's conclusion wouldn't follow because the passage says that the levels of nitrogen in the blood and bone samples were the same. If the levels of nitrogen in blood are three times as concentrated as they would be in bone, that would mess up the conclusion. Is that right?

Also, what would be an example of an answer choice that would discuss the rate in a way that explains the integrity of the information rather than a rate qua rate kind of way? Maybe if it also said that the inferred rate could suggest that the bone concentration wouldn't match the blood concentration, but actually in this case it shows that the blood and bone concentrations do match?