Q19 - According to some astronomers, Earth

[mrudula_2005]

I see why it's C but what is so wrong with B?

The author did presume that the probability of a meteorite striking in the near future is not affected by whether (or not) that (the meteor striking) has occurred during a period in which it would have been expected to occur.

In other words, before stating her intermediate conclusion "so we can expect that Earth will be struck by such a meteorite in the near future", the author doesn't take into account the fact that, "wait, maybe in the past when it was expected that a meteorite strike was coming, it didn't arrive according to schedule")

any insight would be appreciated. thanks!

[giladedelman]

Thanks for posting!

I'm having a hard time following your reasoning. Let's look at the core:

P1: Meteorite hits on average once per 100 million years
P2: Last such meteorite hit 100 million years ago
--->Intermediate C: Meteorite will hit in the near future
--->C: We should fund research on meteorite defense.

The argument assumes that if something happens on average every X years, then X years after it last happened, we can expect it to happen again. This is like sports fans who think their team is "due" to win because they've lost four in a row. (As you said, answer (C) identifies this assumption.)

But how does "whether the event has occurred during a period in which it would be expected to occur" come into play here? Since we're talking about a flaw in the argument, we're talking about a necessary assumption. So let's try negating (B):

"Whether the event has occurred during a period in which it would be expected to occur does affect the probability of a chance event's occurring."

Did negating the assumption mess up the argument? No way! For starters, we don't know anything about whether an even has occurred when expected, so it's already out of scope. For another thing, what does "affect the probability" mean? Couldn't that mean it makes it more likely? So answer (B) really has no bearing on the argument.

Does that make sense to you?

[rbolden]

Can you explain what is wrong with answer choice A?

[cyruswhittaker]

Can you explain what is wrong with answer choice A?

The author makes a "bold prescription," but this answer choice isn't justified because of the second part of the answer: "on the basis of evidence...high probability.."

The argument does not establish a "high probability" for the event, especially considering that it is basing its justification on a time span that involves an average of 100 million years! (And of course we know that we should be very wary of assuming that past events repeat).

And even if it WAS shown to have a high probability of occuring, that seems like it would actually support the "bold prescription."

[giladedelman]

Thanks for chiming in! You're exactly right: the problem with this argument isn't the prescription, it's that the argument fails to establish the probability of the event that the prescription is meant to defend against!

[shaina.affatato]

The word 'Specific' threw me off too on this question. I didn't feel that 'near future' was specific either, but I guess in the scheme of 100 million years, 'near' is technically somewhat specific, at least in LSAT language!

[zainrizvi]

Isn't (B) a 180? the argument DOES assume the probability of a chance event occurring(strike) IS affected by whether the event has occurred during a period in which it would be expected to occur(in the average of every 100 millon years).. It assumes that the event is MORE likely in the future because it hasn't occurred in the expected period.

If we removed the "never" from the original B answer choice, wouldnt it be valid?

[ManhattanPrepLSAT1]

Isn't (B) a 180? the argument DOES assume the probability of a chance event occurring(strike) IS affected by whether the event has occurred during a period in which it would be expected to occur(in the average of every 100 millon years).. It assumes that the event is MORE likely in the future because it hasn't occurred in the expected period.

If we removed the "never" from the original B answer choice, wouldnt it be valid?

Absolutely right! The argument assumes that the probability of a chance event's occurring is affected by whether the event has occurred during a period in which it would be expected to occur.

Answer choice (B) can be tempting because it addresses the issue of whether the past occurrence of an event can affect the future occurrence of such an event.

Let's look at the other incorrect answers:

(A) is incorrect in that the evidence does not establish a high probability for a disastrous event.
(B) states the direct opposite of the assumption being made in this argument.
(D) is not true. The argument is not discussing the likelihood of an ice-age but rather the likelihood of being impacted by a meteorite.
(E) is not true. The argument advocates funding to determine whether there is a means to protect our planet, but does not conclude that a means of protecting our planet can indeed be found.

Hope that helps, but let me know if you have further questions here!

[ss_8185]

This is a time shift error. Are these common on recent LSATs?

[ManhattanPrepLSAT1]

It's still an issue that appears occasionally on the LSAT - with about the same frequency today as has been the case in the past. Every couple of tests, you should spot an LR question that directly tests this issue.

[goriano]

It's still an issue that appears occasionally on the LSAT - with about the same frequency today as has been the case in the past. Every couple of tests, you should spot an LR question that directly tests this issue.

I knew the flaw was in terms of moving from the average frequency to making a prediction based on that average.

But (C) states "specific prediction" whereas the stimulus states "near future." The two, in my mind are not synonymous and I used it as a reason to eliminate (C).

(A) was the only attractive answer left. It DOES makes a bold prescription (this CLEARLY warrants funding) and it IS based on evidence (the average frequency) that only establishes a high PROBABILITY (that a meteor strike in the near future is likely) for a disastrous event.

Help?

[ymcho2013]

For answer choice A, isnt the author making a high probability for the meteorite strikes because he's assuming that because its been 100 million years that it should happen soon?

(he's assuming that the probability of the meteorite strike occurring is affected by whether or not the event has occurred/not occurred)

[marcus.v.p.]

The reason I think answer choice C is correct, is because a specific prediction is made. That prediction being:"The near future." It doesn't say, it will likely occur, or probably occur, but "... we can expect that Earth will be struck by such a meteorite in the near future."

LSAT Geeks, is that okay by ya'll???

[braintreeprez]

I, too was more attracted to answer choice A. The thing that put A over C for me was that most LR questions test the 'last' gap in the argument chain - i.e. the gap b/t the intermediate conclusion and final conclusion, when an intermediate conclusion is given.

Given the discussion here, however, I'm seeing that what makes A wrong is the fact that 'high probability' is an assumption in itself. Just because something has happened a certain way before does NOT mean there is a high probability it will proceed that way again.

[dhlim3]

I eliminated A because it is a flaw in the transition from the Intermediate Conclusion and the Main Conclusion. The correct answer should point out the flaw between the PREMISE and the CONCLUSION (Intermediate Conclusion is still a conclusion). I didn't look too much into the "high probability" term issue.

Is my method too risky? So far I have yet to find a flaw problem where the correct answer actually reflected a flaw in the transition from I.C. to Main Conclusion. Correct me if I'm wrong.

[haeeunjee]

For Flaw questions, I know we need to weaken the reasoning rather than premises, which is why I passed by (C) on my first glance because I thought it was trying to weaken the premise/intermediate conclusion "so we can expect Earth to be struck by meteorite in near future." (I ended up choosing (A) because I thought it pointed out a flaw in the reasoning, connecting "bold prediction" in the AC with the "clearly warrants" in the main conclusion.)

But is (C) still right because it's pointing out a flaw between the reasoning of a premise and an *intermediate* conclusion? Even though it's not directly weakening the reasoning for the main conclusion, but weakening the link between the premise "the last such incident was 100 mill yrs ago" and the intermediate conclusion "so we can expect Earth to be struck by meteorite in near future". I guess so, because if intermediate conclusions are weakened, main conclusions are therefore weakened?

If Flaws include weakening intermediate conclusions, that's a significant nuance for me to keep in my mental toolbox...

Thanks for all the great explanations thus far!