In a study conducted over several years, seabird and domesticated cat

Question

In a study conducted over several years, seabird and domesticated cat populations on a geographically isolated island changed from year to year. Researchers found that over the course of the study, the relationship (R) between seabirds and domesticated cats was such that the island's seabird population was three times as likely to decrease from the previous year if the island's domesticated cat population increased (even if slightly) during the same year. The researchers are about to begin a second, follow-up study with the same duration as the first study. Based on recent trends, the researchers made the following projections: R will hold and the island's domesticated cat population will decrease during more years of the second study than it did in their first study.

Assuming that the information above is true, select for Can be inferred as true the statement that can be most reasonably inferred as true from the information provided, and select for Can be inferred as false the statement that can be most reasonably inferred as false from the information provided. Make only two selections, one in each column.

Assuming that the information above is true, select for  Can be inferred as true  the statement that can be most reasonably inferred as true from the information provided, and select for  Can be inferred as false  the statement that can be most reasonably inferred as false from the information provided. Make only two selections, one in each column.

ID: 700201

KarishmaB · Accepted Answer

I would easily classify this question as Hard. A fair bit of struggle involved because of the long statements and complicated relations and comparisons.

Bird and cat pops changed from year to year (so they were not constant. We assume that they changed every year)

Given  R: If cat pop increased, probability of bird pop decreasing became 3 times.

So even if cat pop decreases, there is a pobability that bird pop will also decrease (albeit smaller).

Predictions:
  - R  will hold
- Cat pop will decrease during more years of the second study than it did in their first study 
 (IMPORTANT POINT -   COMPARISON between first study and second study , not between the number of years of increase/decrease in either one study alone)

Now we choose for "Can be inferred to be true" and "Can be inferred to be false"

If the researchers' projections are accurate, the island's seabird population is likely to increase during more years of the second study than it did in the first.

Predictions are accurate. We know that in second study, cat pop will decrease in more years than in first study.
So the cat pop will increase or stay the same in fewer years in the second study than in the first study. The probability of bird pop decreasing becomes much greater (3 times) in these years, and there will be fewer of these years. So, we are likely to see fewer years of bird pop decreasing in second study than in first study. So we are likely to see more years of bird pop increasing in second study than in first study. 
The word "likely" gives me a lot of confidence in this option.  ANSWER

During the first study, most years had an increase in the island's domesticated cat population.

As we said before, we do not know how the split was between "no of years of cat pop increases" and "no of years of cat pop decreases" in the first study. There is no such comparison given.

If the researchers' projections are accurate, the island's seabird population is likely to increase during most of the years of the second study.

As we said before, we do not know how the split was between "no of years of bird pop increases" and "no of years of bird pop decreases" in the first study and hence nothing can be said about second study either. There is no such comparison given.

During the first study, the island's seabird population decreased only when the island's domesticated cat population increased.

We know that there is a probability of bird pop decreasing even when cat pop decreases. That probability is not 0. It becomes 3 times when cat pop increases. Hence we know that "cat pop increase" is not necessary for "bird pop decrease".  ANSWER 
Hence this statement can be inferred to be false.

During the first study, most years during which the island's seabird population decreased were years during which the island's domesticated cat population increased.

Cannot say whether it is true or false. It is natural that it could be true but it could be false also. e.g. what if out of 20 years of first study, in 18 years cat pop decreased and in 2 years cat pop increased. Out of those 18 years, bird pop decreased in 6 years but in both years of cat pop increase, bird pop decreased. This is possible.

PReciSioN · Answer

Given 1) Relation R - During the first study, SB was 3x as likely to decrease if Cat increased. This means if there were x years when the Cat population decreased AND the SB population decreased, then there were 3x years when the cat population increased AND SB decreased.
2) year to year changes, so only increase or decrease possible.
3) Second study has same no of years and R will hold.
4) More no of years with cat population decreasing.

Now, while I agree that option D MUST BE FALSE, I initially thought it is not correct that A must be true.

One example I came up with to show this ->

On thinking a little deeper, I now don't agree with the red highlighted part in my statement 1 at the top. Rather I feel that the likelihood of SB decrease if Cat increase is linked to no of years with Cat increase & SB decrease divided by no of years with cat increase or in study 1 (3/4) and not simply 3 as I was initially assuming.

Just posting this for someone who might have been thinking along the same lines as I was initially.

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jaky_nguyen · Answer

Hi PReciSioN , There is one thing I concern: As you mention '2) year to year changes, so only increase or decrease possible.' This only applies to the 1st study, while the projection doesn't mention anything about this in 2nd study. So it could be that there would be years in which S or C will be unchanged in 2nd study. Posted from my mobile device GMAT-Club-Forum-s36ewgf2.png

PReciSioN · Answer

Yes jaky_nguyen , it is not mentioned that the second study has only increase/ decrease in the populations on a year to year basis. It might have populations which are same accross years. But this strenthens our answer of A even more. Given that the number of years with decreasing cat populations are given to be less than original study, and now if we consider that some years the population might be same, this means the number of years where cat population increases is even less. Since during years of increasing cat population, SB population is 3 times more likely to decrease, this implies that in the new study SB population will NOT decrease for more years than in original study. Now this NOT DECREASE can be via an increase or via population remaining same but I feel that it would be unreasonable to assume that the increase in the number of NOT DECREASE SB years is purely due to the SB population remaining the same. It is likely (again in the words of the option-A) that SB population increased for more number of years. GMAT-Club-Forum-jrej58co.png

jaky_nguyen · Answer

Thank you for clarifying my concern but I can't find any in Paragraph supporting this 'I feel that it would be unreasonable to assume that the increase in the number of NOT DECREASE SB years is purely due to the SB population remaining the same.' Also consider my example below, option A can't be infered as true https://gmatclub.com/forum/download/file.php?id=81756 And looking at both my example and yours, option E can be infered as true: 'During the first study, most years during which the island's seabird population decreased were years during which the island's domesticated cat population increased.' 3 years with increase in C out of 4 years decrease in SB GMAT-Club-Forum-wwjh9xj1.png GMAT-Club-Forum-m975j3b9.png

PReciSioN · Answer

Hi jaky_nguyen , Like I mentioned in my original post, we should not associate the likelihood of SB decrease if cat increases with the absolute number of years in which SB decreases AND cat increases. Rather the likelihood should be the ratio. In your example, in study -1 , the likelihood (probability) of SB decrease if cat increases (this is conditional probability) is 3/7 (3 years of SB decrease & cat increase divided by 7 years in which cat increased.) This probaility of 3/7 should be 3 times the probability of SB decrease if cat decrease which is not the case in your example (and it wasnt the case in my original example as well.). Furthermore, in your second example likelihood of SB decrease if cat increase is 6/6 = 1. In your example, the relation R given by does not hold. As for why E is incorrect. take this example. Suppose in study 1, cat increased for 10 years and for 3 of those 10 years, SB decreased. So this probability is 3/10. As per relation R, the probability of SB decrease if Cat decrease is 1/10 (3 times less). This can be the case if cat decreased for 100 years, and for 10 of those 100 years SB decreased. As you can see in this case the majority of years in which SB decreases is when cat decreases, but still the likelihood of SB decreasing is more when cat increases simply due to the fact that the number of years in which cat decreases is much more. GMAT-Club-Forum-0bgezauu.png

jaky_nguyen · Answer

Hi  KarishmaB ,

I have a question regarding to option 1: "If the researchers' projections are accurate, the island's seabird population is likely to increase during more years of the second study than it did in the first."

The predictions in study 2 don't mention Bird and cat pops changed from year to year as in study 1. Therefore, Bird and cat pops could be unchanged in study 2.

Considering my below example:

+ Study 1:
Cat: 6 yrs with increase; SB: 3 yrs increase + 3 yrs decrease
Cat: 6 years with decrease; SB: 5 yrs increase + 1 yrs decrease
+ Study 2:
Cat: 3 yrs with increase; SB: 2 yrs increase + 1 yrs decrease
Cat: 9 years with decrease; SB: 5 yrs increase + 3 yrs unchanged + 1 yrs decrease

=> while bird in study 1 increased in 8 years, bird in study 2 increased in 7 years, fewer than in study 1.

Does this mean this option could be true or false?

egmat · Answer

Hi  jaky_nguyen

Happy to chip in. I believe your example goes slightly against the data given. Allow me to explain:

R(relationship) = The island's seabird population was three times as likely to  decrease  from the previous year if the island's domesticated cat population  increased  (even if slightly) during the same year.

So, as per your example of Study 1: 
Cat: 6 years with increase; SB: 6 years decrease (3 times as likely as when Cat population does not increase even slightly)
Cat: 6 years without increase (means Cat population decreases because population changes year to year); SB: Uncertain. Might still increase or decrease depending on how much Cat pop decreases. (Remember even a slight increase in Cat population makes a decrease in Seabird population 3 times as likely. So, a decrease in Cat population could still lead to a decrease in SB population, albeit at a lower likelihood.)

Now, for Study 2: 
The passage says that R holds. This means we still find the same relationship as above. So...
Cat: 3 years with increase; SB: All three years with decrease (SB increase during these years is out of the question)
Cat: 9 years with decrease; SB: Again uncertain, as above. Might still increase or decrease depending on how much Cat population decreases.

So, how is Statement A true? 
 
 Let's start with R: 
The island's seabird population was three times as likely to  decrease  from the previous year if the island's domesticated cat population  increased  (even if slightly) during the same year.

Now, the predictions are: 
 
 R will hold 
 The island's domesticated cat population  will decrease during more years  of the second study than it did in the first study.

Now, if we flip prediction 2, we get: 
The island's domesticated cat population  will increase during fewer years  of the second study than it did in the first study.

Now, if we apply "R", we get: 
The islands seabird population  will decrease during fewer years  of the second study than it did in the first study.

Now, if we flip the above sentence, we get: 
The islands seabird population  will increase  during  more years  of the second study than it did in the first study. 
This is the same as choice A: If the researchers' projections are accurate, the island's seabird population is  likely to increase  during  more years  of the second study than it did in the first.

Hope this clarifies.
Happy learning!

-Abhishek

Dino_Dhamaka · Answer

Evaluate

RO1,  Can be inferred as true :

In each year of a study that was conducted over a period of several years, the populations of both seabirds and domesticated cats on a geographically isolated island changed—that is, the populations either increased or decreased each year from the previous year’s numbers. The researchers found that a certain relationship  R  held: the island’s seabird population was three times as likely to decrease from the previous year if the island’s domesticated cat population increased even slightly during the same year. Because the populations of each of the two types of animal changed each year, we can infer that when the domesticated cat population decreased, the seabird population was only one third as likely to increase as when the cat population decreased. It also follows, then, that the seabird population was more likely to increase when the cat population decreased than when it increased. So, if the researchers are correct that  R  will hold during the second, follow-up study that will last the same number of years as the first study, and that the domesticated cat population will decrease during more years than it did during the first study, then it can reasonably be inferred that the seabird population will increase in more years than it did in the first study.

The correct answer is  If the researcher’s projections are accurate, the island’s seabird population is likely to increase during more years of the second study than it did in the first .

RO2,  Can be inferred as false :

If it was determined in the first study that the island’s seabird population was three times as likely to decrease in years when the island’s domesticated cat population increased, then it cannot be true that the island’s seabird’s population  never  decreased in years when the island’s domesticated cat population did not increase. To see this, consider that for  R  to hold, the number of years  x  in which the island’s cat population increased and the island’s seabird population decreased has to be three times as great as the number of years  y  in which the island’s cat population did  not  increase and the island’s seabird population decreased. In other words, ​​ = 3. And if ​​ = 3, then  y  cannot equal 0. Therefore, it must NOT be the case that, during the first study, the island’s seabird population decreased only when the island’s cat population increased.

The correct answer is  During the first study, the island’s seabird population decreased only when the island’s cat population increased .

Ziniya · Answer

KarishmaB  , I understand why eliminated B & C. we are only given information What will happen when cat pop increases. There is no information regarding what happens when cat pop decreases. Is this correct to infer this inversely proportion relationship?

KarishmaB · Answer

The relation given in B and C is different from what is discussed in the argument.

The argument discusses "No of years cat pop decreased in  first study " VS "No of years cat pop decreased in  second study "

Option (B) discusses "No of years cat pop increased in  first study " VS "No of years cat pop decreased in  first study "

Option (C) discusses "No of years bird pop increased in  second study " VS "No of years cat pop decreased in  second study "

So (B) and (C) have faulty comparisons that are out of scope for us.

PeachSnapple1 · Answer

In my experience, each DI question will have a nuance that you need to be aware of if you want to the get that question correct. If you miss it, then you will have a very rough time.

The nuance of this question lies in this sentence    , specifically the phrase   .  Three times than what? Apparently it means three times as likely as to increase  even if  the cat pop. increase. So basically, when cat pop increase, there will be two possibilities: bird pop decrease or increase. So, either of those probability can equal 0%, because 0% times 3 still equal 0%. Therefore,   During the first study, the island's seabird population decreased   only when     the island's domesticated cat population increased.]  must be false.

Not gonna lie, I spent the whole 15 minutes just to catch that nuance, as I had eliminated all answers (except for the first one as I know it is a must-be-true statement. These questions give me headache to be honest.

consistentprep · Answer

Things to keep in mind:
1) use of word "than"
2) use of word "more" vs "most"
3) use of word "only" - extreme word

Why option E cannot be inferred? 
If cat population increased, then the chances of seabird population decreasing were 3x higher than if cat population didn’t increase.
Just because seabird population is more likely to decrease when cats increase, doesn’t mean that most seabird decreases happened when cats increased.
Why?
 Imagine the following 10-year study: 
Cat population increased in 2 years, stayed the same or decreased in 8 years.
In the 2 years when cats increased, seabirds decreased both times.
In the 8 other years, seabirds still decreased 3 times.
So total seabird decreases = 5:
2 of them happened in cat-increase years.
3 of them happened in other years.
So in this case: Most seabird decreases (3 out of 5) happened in years without a cat increase.

Why option C is wrong ? 
"Most of the years" is a stronger claim than just "more than before."
The projection only says "more years" than in the first study, not necessarily "most".
So this goes beyond what’s stated.

Why option D is correct ? 
Use of Only makes it an extreme statement and narrows down the scope to single case. Hence  D cannot be inferred  from the argument

avatar0980 · Answer

for option E, if R is the relationship and not simply correlation how is it possible that in some years if cat pop decreases, bird pop can increase? even if we assume it to be correlation, would the word 'most years' and not 'all years' make it possible to select this option as ' Can be inferred as true'

egmat · Answer

avatar0980  I like your question. You've identified something important: Statement E  is  logically reasonable, but it's not the  best  inference. Let me explain why Statement E isn't selected despite making logical sense.

Why Statement E Is Logically Sound (But Not Selected)

You're absolutely right that Statement E could be defended:

"During the first study,  most years during which the island's seabird population decreased  were years during which the island's domesticated cat population increased."

The Logic: 
 
 Relationship  R  states: Seabirds are  3 x  more likely  to decrease when cats increase 
 If  P (seabird decrease | cat doesn't increase)  = p , then  P (seabird decrease | cat increase)  = 3p  
 Therefore, among all years when seabirds decreased, years with cat increases would contribute  3 x as many seabird-decrease events 
 This suggests  most  seabird-decrease years would coincide with cat-increase years

Your observation is correct: the word "most" (not "all") makes this probabilistically reasonable.

So Why Isn't Statement E the Answer?

The question asks for what can be  most reasonably inferred —this requires comparing strength of inferences:

Statement 1 (The Correct "TRUE" Answer): 
"If the researchers' projections are accurate, the island's seabird population is likely to increase during  more years  of the second study than it did in the first."

Direct inference from projections:  Cat population will decrease during MORE years in second study 
 Fewer years with cat increases → Fewer years with  3 x likelihood of seabird decrease 
 Conclusion follows  directly  from stated projections with minimal assumptions

Statement E: 
 
 Requires  probabilistic reasoning  about distribution of events 
 Makes  implicit assumptions  about: (1) How many years had cat increases in first study, (2) Base rate of seabird decreases when cats don't increase 
 While logically defensible, it's a  weaker inference  than Statement 1

The Key Distinction:

Statement E could be true  if certain distributional assumptions hold , but Statement 1  must be more likely true given the explicit projections . On GMAT inference questions,  choose the most direct inference  that requires the fewest additional assumptions.

Addressing Your Specific Questions:

R  only specifies  one direction : what happens when cats increase (seabirds  3 x more likely to decrease).  R  says  nothing  about what happens when cats decrease. When cats decrease, seabirds might increase, decrease (at lower probability), or remain stable—we simply don't know from  R  alone.

Yes, "most" makes Statement E  logically reasonable , but in a comparative evaluation, Statement 1 is still  more directly inferable  from the given information. That's why Statement 1 is selected.

I hope this addresses your questions clearly? If you still have any doubt, feel free to follow-up.

KarishmaB · Answer

avatar0980  - No, option (E) is incorrect. You cannot infer it to be true.

R only tells us that whatever is the probability of bird population decreasing when cats are same or decreasing, that probability becomes 3 times when cat population increases. 
So say Bird decreasing is 0.3 when cat is steady or decreasing. Bird decreasing becomes 0.9 when cat increasing.

(E) During the first study, most years during which the island's seabird population decreased were years during which the island's domesticated cat population increased.

Not necessarily true. 
Say 50 years study. 
40 years cat decreased - bird decreased in 12 years
10 years cat increased - bird decreased in 9 years

Out of total 21 years of bird decrease, cats increased in only 9 years. Hence cannot infer statement to be true.

In a study conducted over several years, seabird and domesticated cat

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Can be inferred as true	Can be inferred as false
		If the researchers' projections are accurate, the island's seabird population is likely to increase during more years of the second study than it did in the first.
		During the first study, most years had an increase in the island's domesticated cat population.
		If the researchers' projections are accurate, the island's seabird population is likely to increase during most of the years of the second study.
		During the first study, the island's seabird population decreased only when the island's domesticated cat population increased.
		During the first study, most years during which the island's seabird population decreased were years during which the island's domesticated cat population increased.

GMAT Two-part Analysis (TPA) Questions

­In a study conducted over several years, seabird and domesticated cat

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In a study conducted over several years, seabird and domesticated cat