A chemical company claims that, since only one of 520 rats that were

jainu - you're reading far too much into answer D. It says that *certain* (i.e. 'some', not 'all') substances which are carcinogenic for humans are not carcinogenic for rats. That means that some substances which are carcinogenic for humans *are* also carcinogenic for rats. In any case, if there were no relationship between rats and humans, the study in the passage would be completely pointless, and wouldn't prove anything about whether the sweetener was dangerous for humans, so D would weaken the argument.

The answer should be B - if 1 in 540 rats will develop cancer spontaneously, statistically you'd expect one of the rats in the study to develop cancer whether given the sweetener or not. This suggests that the sweetener may not have caused a single case of cancer, strengthening the claim that the sweetener is "not carcinogenic". If we didn't have this information, and if the one cancer from the study were indeed attributable to the sweetener, then clearly the sweetener *is* carcinogenic, at least in a small proportion of cases. For the company to make the rather extreme claim that the sweetener is "not carcinogenic", it must be able to establish that the sweetener is not responsible for a single case of cancer, so we need an answer choice which provides an alternative explanation for the one observed case of cancer in the study.

The argument says, essentially:

We tested our sweetener on rats, and one rat got cancer. Therefore our sweetener does not cause cancer.

That should seem like a baffling argument, on first read. The conclusion seems to contradict the evidence. It is only a good argument if we have a better explanation, different from the sweetener, for the one observed case of cancer. And if it would be normal, when 500 rats do not consume the sweetener, for one of them to get cancer, then the experiment's results are exactly what you'd expect if the sweetener does not cause cancer. If the sweetener did cause cancer, you'd expect more than 1 out of 500 rats to get cancer after taking it. That's why we care a lot here about the 'spontaneous incidence' of cancer among rats in this question.

The logical issue tested here is actually topical right now. If, after one million vaccine shots are administered, three people experience some health effect, there's no way to know if that's alarming or normal unless we know how many people would experience that same health effect without vaccination. Just because one thing happens after another does not mean the first thing caused the second -- that's the "post hoc ergo propter hoc" fallacy in logic.

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Conclusion here is that the sweetner will not cause cancer in humans since only 1/520 rats got cancer in a trial on 520 rats. The questions asks what will support this.

B

A chemical comp claims that, since only one of 520 rats that were given high doses of a new artificial sweetner developed cancer while all the others remained healthy, the sweetner is not carcinogenic for human beings and ought to be approved for consumption for human consumption.

Which of the following, if true, most strongly supports the chemical company's claim?

the first assumption: one of 520 is not carcinogenic.
the second assumption: not carcinogenic for rats means not carcinogenic for human.

(A)Chemicals that are carcinogenic for rats are usually also carcinogenic for other animals, such as guinea pigs, used in experiments.
irrelevant.Two flaws: "animals" instead of "human", "carcinogenic" instead of "not carcinogenic".

(B)The spontaneous incidence of cancer in this particular strain of rat is approx one in 540.
it is the first assumption.

(C)Tests conducted on a certain strain of mouse show that, of 500 mice given a dose of sweetner similar to that of rats, 53 developed cancer.
irrelevant. Is 53 of 500 normal?

(D)Certan chemicals that are carcinogenic for human beings have been shown not to be carcinogenic for rats.
weaken

(E)The average lifespan of the strain of rat used in the experiment when the rats wrer 13 months old.
irrelevant.

i think D because if it is proved that chemicals carcinogenic for humans are not carcinogenic for rats. This means that carcinogens for humans are exclusive to carcinogens for rats. That is there is no common chemical that can be carcinogenic for both. Hence chemicals that have been found in arg to be carcinogenic for rats are not carcinogenic for humans. Hence can be approved.
Also,
If carcogenic for humans ---> not carcogenic for rats
reverse the relation....Carcogenic for rats -----> not carcogenic for humans.So clealry D
So D

If naturally 1 out of 540 mice get this , then 1 mice that got cancer in studies was not because of sweetener but natural cause. that is what given at B.

(B)The spontaneous incidence of cancer in this particular strain of rat is approx one in 540.
it is the first assumption.

Even in the increased sample size of rats, there is still just 1 rat who got Cancer, this proves the point that the rat has caught cancer for some other reason than sweetner. Hence B is correct.

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B. How B is irrelevant , we don’t care about spontaneous. What if cancer incidence happens later
C . so if it is 53 in some strain of rat then it can be dangerous for humans also
D. it weakens the claim . it maybe possible that whatever doesn’t appear in rat experiments, it may happen to humans. too risky
E. So It means the experiment was not completed
I chose A because if something happen to rats then it happens to other animals. So chances are it happens to humans also. In other side, if something doesn’t happen to rats, it may not happen to animals and thus may not happen to humans.
That’s why I chose A
But I find I am too far from correct

VeritasKarishma IanStewart DmitryFarber BrentGMATPrepNow CrackVerbalGMAT EducationAisle vv65: what am I missing in understanding the argument.

thanks!

I was about to explain by using the example of Covid-19 vaccines, but IanStewart has done that already

The right answer will show that if only 1 of the 520 rats got cancer, that is good news. The right answer will make the statement "the sweetener is not carcinogenic for humans" more believable.

Spontaneous means "happening without apparent external cause; self-generated"
What Answer choice B means: for this type of rat, 1 in 540 rats usually get cancer anyway. In a test of whether the sweetener is harmful, we can expect 1 in 540 rats to get cancer. Suppose 50 rats in the group had developed cancer, it would have been worrying. But 1 in 520 is not cause for worry.

About your question "What if cancer incidence happens later?"
It is true that we do not know how long the testers waited to see the effects of the sweetener. Obviously, if they waited (say) only two days, that is too short and cancer could develop later. But we cannot assume that they did not wait long enough. In the absence of other information, we assume that things were done normally.

[Digression: The Covid-19 vaccines were tested for some months, not for many years. We do not know about long-term effects of the vaccines. But we do not know the long-term effects of the Covid-19 infection either, and we do know that the short term effects of the infection are deadly. So the whole world needs to be vaccinated ASAP]


This is irrelevant. It neither strengthens nor weakens the conclusion that the sweetener is not carcinogenic for either rats or humans or any other animals.
You said, "if something happen to rats then it happens to other animals."
Yes, that is the reason for testing on rats. And the main purpose of testing is to find out whether the sweetener is carcinogenic for rats. Answer Choice A tells us nothing about that.

Important to remember:
It is possible that the sweetener is carcinogenic despite the info in Answer Choice B. But the right answer of a strengthen question does not need to prove the conclusion beyond any doubt. It just needs to make the conclusion more believable.

Hope this helps. Thank you for asking me!
_________________

mSKR - Ian has already explained why (B) works so nothing left for me to add on that.

I am giving my analysis of the other options below.

Company's Claim: The sweetener was given to 520 rats and only 1 got cancer.
Company's Conclusion: The sweetener is not carcinogenic for human beings and ought to be approved for
human consumption.

There are multiple things I need to know before I can say whether the company's claim is valid:

1. Do rats normally get cancer? If no, then why did that 1 rat get it? Perhaps the sweetener is carcinogenic then. If yes, what is the normal rate of occurrence in rats? If 1 in 10,000 rats get cancer then the sweetener may have had a bad effect. If 1 in 20 rats get cancer then the sweetener may actually be acting as anti-cancer! If 1 in 500 rats gets cancer then it seems the sweetener has no link to cancer.
2. Do human beings also experience the same effect of chemicals as do rats? If yes, then it seems the company's conclusion is justified. If no, then perhaps the company's conclusion is not justified.

We need something that supports the company's claim i.e. something that says that the sweetener is not carcinogenic.

(A) Chemicals that are carcinogenic for rats are usually also carcinogenic for other animals, such as guinea pigs, used in experiments.

This tells us that other animals used in experiments also experience effects similar to rats. But what about humans? How similar or different their reactions are? The similarity between all animals that are experimented on is irrelevant to us. We need to know the differences/similarity between those animals and humans.

(B) The spontaneous incidence of cancer in this particular strain of rat is approximately one in 540.

Correct. As discussed above, we were looking for this information. If naturally 1 in 540 rats gets cancer then it looks like the sweetener is not linked to cancer.

(C) Tests conducted on a certain strain of mouse show that, of 500 mice given a dose of sweetener similar to that given the rats, 53 developed cancer.

Are mice similar to humans? If they are, then this weakens our conclusion.

(D) Certain chemicals that are carcinogenic for human beings have been shown not to be carcinogenic for rats.

So this says that rats and humans are not very similar. Some chemicals could be carcinogenic in humans even if they are not in rats. Then this weakens the company's conclusion.

(E) The average lifespan of the strain of rat used in the experiment is 2 years the chemical company terminated the experiment when the rats were 13 months old.

This again puts a question mark on the company's conclusion. The rats could have developed cancer later. Hence, it doesn't strengthen the conclusion.

Answer (B)

A – this doesn’t strengthen the chemical company’s claim – we don’t care about ‘other animals’.

B – CORRECT. If the incidence of cancer in this strain is one in 540, then we have reason to believe the sweetener is not carcinogenic for human beings – that one case would be attributable to the spontaneous incidence of cancer in rats.

C – Opposite direction – weakens the chemical company’s claim.

D – Opposite direction – suggest that results from the test of 520 rats may not be accurate for human beings. If certain chemicals that are carcinogenic for human beings are not carcinogenic for rats, then the results could potentially be less accurate for human beings.

E – Out of scope. Lifespan of the strain of rat is not relevant here.
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Someone please help me with my analysis

Reason to select option A: If it has worked on other animals which are normally used for experiments, then it will work on humans too.

Reason to not select option B: The particular strain of rats normally do not get infected with cancer. So probably, getting infected with cancer due to any other factor is also not likely. That doesn't conclusively prove that humans too might not get infected.

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wrt B, in the experiment the odds of getting cancer improved from 1/540 to 1/520. don't we need to know if this increase is statistically significant or not? are we allowed an assumption for an assumption?

The difference between 1 in 540 and 1 in 520 is negligible. 500 is a big number compared with 1.
If the stats were 1 is 3 people, then 1 in 4 is significantly different. But since we are talking about 1 in 500, 520 or 540 doesn't make much of a difference.
In any case, we have no other option that strengthens the claim.

Say you have 540 rats, exactly one of which will get cancer. You line the rats up and pick the first 520 of them for your experiment. The probability the rat that will get cancer is among the 520 you choose is 520/540 ~ 96.3%.. So it's enormously probable you'll have a rat among your 520 that will spontaneously develop cancer, and you don't need any information about statistical significance.

One way to look at B:

At the time of performing the experiment, the company knew that the baseline cancer rate was approximately 1/540 for this particular strain of rats.

Based on this info, the company expected one of the two results:

A. If the results show an increase over the baseline rate, then the sweetener is likely a factor, making it unsafe

B. If the results show no increase over the baseline rate, then the sweetener has no impact, and therefore safe

@IanStewart
first off... wonderful explanation given by you!!!
Just one query for option B in this question how did you assume that spontaneous incidence of cancer means the rat will get cancer no matter whether they eat the sweetner or not?

I interpreted this option as the incidence of spontaneous carcinogenic effects of the sweetner is only 1 in 540

Option elimination -

(A) Chemicals that are carcinogenic for rats are usually also carcinogenic for other animals, such as guinea pigs, used in experiments. Other animals are out of scope.

(B) The spontaneous incidence of cancer in this particular strain of rat is approximately one in 540. - Spontaneous means without external stimulus but even if we don't know what spontaneous means (it's an adjective) it says the incidence of cancer in this strain of rat is 1 in 540, which means that even without this sweetener, 1 in 540 rats develop cancer. It actually strengthens our case.

(C) Tests conducted on a certain strain of mouse show that, of 500 mice given a dose of sweetener similar to that given the rats, 53 developed cancer. - 53 out of 500 is more than 10%. Pretty significant jump from 1 in 520 <0.2% . From 0.2% to 10% raises doubts. At best it is a weakener.

(D) Certain chemicals that are carcinogenic for human beings have been shown not to be carcinogenic for rats. - Weakener.

(E) The average lifespan of the strain of rat used in the experiment is 2 years the chemical company terminated the experiment when the rats were 13 months old. Weakener.

In Option B,

The spontaneous probability of cancer in rats is 1 in 540 and it is 1 in 520 in the sweetener test batch. Doesn't it mean it the sweetener increased the probability of cancer in rats?