#25 - Each major earthquake in this region has been preceded

[Administrator]

Please post your questions below!

[sodomojo]

Why is (B) incorrect?

The flaw here seems to be a time shift error, with the inference that because a preceding event has occurred, the correlated event will also occur.

Both (B) and (E) seem to be matches for this flaw.

[James Finch]

Hi Sodomojo,

It looks like you misidentified the flaw in this question. This stimulus actually contains an error in conditional reasoning, specifically a Mistaken Reversal, not a time shift error, because nothing in the stimulus says that there haven't been tremors without a following earthquake. The premise given is that all recent earthquakes have been preceded by tremors (Earthquakes Tremors), and then concludes from that fact that because we now have tremors, there will be an earthquake (Tremors Earthquake). So what we're actually looking for is an answer choice that has a Mistaken Reversal in it, not a time shift error.

Answer choice (B) says that when there is high snowfall, then the river overflows during the spring thaw (Snowfall Overflow), and concludes that because there was record high snowfall this year, the river will overflow again next spring (Snowfall Overflow). In fact, this answer choice has no flaw, it simply applies the conditional logic set up by its premise to the actual circumstances correctly, which makes it an incorrect answer choice.

(E) says that disease among humans is preceded by high infection rate in animals (Human Disease Animal Infection) ("preceded by" is a hint here), concluding from that information that because we now have high infection rate amongst animals, we will soon have disease in humans (Animal Infection Human Disease) This is the Mistaken Reversal we were looking for, an exact match for the flawed reasoning present in the stimulus, and thus the correct answer choice.

Hope this helps!

[eatmycoinpurse12]

Can you please explain a/c A??

I want to see how it's diagramed. Originally I had TS > H ....but is it H>TS ??

Because the reasoning is ~TS > ~H


MORE IMPORTANTLY, I don't understand how to diagram E for the life of me. How does "occurred soon after" mean preceded by (come before)?

I was thinking temporal, which is wrong, but I still can't see how:
Local outbreak can lead to high infection.

I thought ALL local outbreaks occurring soon after the detection high infection meant if you see a high infection then you KNOW it will be followed by local outbreak. How is this read as high infection being a requirement for local outbreak?

[Claire Horan]

Ah, this is a difficult one!

Okay, as James Finch explained in an earlier post:

This stimulus actually contains an error in conditional reasoning, specifically a Mistaken Reversal, not a time shift error, because nothing in the stimulus says that there haven't been tremors without a following earthquake. The premise given is that all recent earthquakes have been preceded by tremors (Earthquakes Tremors), and then concludes from that fact that because we now have tremors, there will be an earthquake (Tremors Earthquake). So what we're actually looking for is an answer choice that has a Mistaken Reversal in it...

(A) suggests this diagramming:
Premise 1 of the past says that every time there was a tropical storm there was a major hurricane:
TS MH
Premise 2: TS
Conclusion: MH

This reasoning is flawed as a Mistaken Negation rather than a Mistaken Reversal. It suggests that when the sufficient condition is absent, the necessary condition can't occur. But it may be that there are other causes of major hurricanes. Compare the situation that every time my cat eats a plant he vomits. But that doesn't mean he won't also vomit when he eats something else besides a plant. (He will.)

(E) is diagrammed as follows:
Premise 1: OD DHIR
Premise 2: DHIR
Conclusion: OD

This is a classic Mistaken Reversal. The flawed logical step was: DHIR OD.

I think the trouble you are having with (E) is that you are assuming an if-then relationship must be a before-after relationship. It can be that way but doesn't need to be.

(E) states that, in the past, if there was an outbreak, then there was detection of high infection rates beforehand. Remember that the arrow only means that knowing that the outbreak happened is sufficient to know that there must have been the detection of high infection rates. The arrow doesn't tell us what came first.

This excerpt is actually a perfect example of a Mistaken Reversal, so let's break it down:

I thought ALL local outbreaks occurring soon after the detection high infection meant if you see a high infection then you KNOW it will be followed by local outbreak. How is this read as high infection being a requirement for local outbreak?

Just because every time there was an outbreak there was a high infection rate before that does not mean that there was an outbreak EVERY time there was a high infection rate. Compare: "Every time I get a cold, it's the day after I haven't slept enough." That doesn't mean that every time I don't sleep enough I get a cold. It just means (the contrapositive of the statement) that if I DO sleep enough, I don't get a cold.

I hope this helps!

[hlee18]

Hi Sodomojo,

It looks like you misidentified the flaw in this question. This stimulus actually contains an error in conditional reasoning, specifically a Mistaken Reversal, not a time shift error, because nothing in the stimulus says that there haven't been tremors without a following earthquake. The premise given is that all recent earthquakes have been preceded by tremors (Earthquakes Tremors), and then concludes from that fact that because we now have tremors, there will be an earthquake (Tremors Earthquake). So what we're actually looking for is an answer choice that has a Mistaken Reversal in it, not a time shift error.

Answer choice (B) says that when there is high snowfall, then the river overflows during the spring thaw (Snowfall Overflow), and concludes that because there was record high snowfall this year, the river will overflow again next spring (Snowfall Overflow). In fact, this answer choice has no flaw, it simply applies the conditional logic set up by its premise to the actual circumstances correctly, which makes it an incorrect answer choice.

(E) says that disease among humans is preceded by high infection rate in animals (Human Disease Animal Infection) ("preceded by" is a hint here), concluding from that information that because we now have high infection rate amongst animals, we will soon have disease in humans (Animal Infection Human Disease) This is the Mistaken Reversal we were looking for, an exact match for the flawed reasoning present in the stimulus, and thus the correct answer choice.

Hope this helps!

Hi James,

I am confused on how the sufficient versus necessary conditions are set up here... I understand how the question stem is categorized as the following: Major earthquake Series of minor tremors. Series of minor tremors Major earthquake, hence the Mistaken reversal.

But if I try to do this for B, I get River overflowed in spring Winter with high snow. Winter with high snow River overflow in spring.

In comparing "preceded by" from the question stem and "following" from B, I would think that the order of which the sufficient and necessary conditions are placed would be the same...

[James Finch]

Hi H. Lee,

It looks like you confused the sufficient and necessary conditions in (B). The wording in the answer choices is meant to be confusing, and does take a moment to suss out. In (B), the sufficient condition indicator is "every," which attaches to the condition of "winter with high snowfall" ("spring thaw" is put in there as a red herring). So having a "winter with high snowfall" becomes a sufficient condition for the other occurrence, the river overflowing, or:

Winter with High Snowfall River Overflows

Then we're given that the latest winter was exceptionally snowy, from which we can validly conclude that river will overflow after the spring thaw. There's no flaw in (B), which makes it an incorrect answer choice for this Parallel Flaw question.

Hope this clears things up!

[hlee18]

Hi James,

Thank you for the clarification. This prompts me to better memorize the list of sufficient and necessary condition list!

[VamosRafa19]

Can B be eliminated from the fact that it says 'record amount of snow' which could be a record high or record low?

[Jeremy Press]

Hi Rafa,

That's an interesting suggestion. Let me say two things: first, I don't think I personally would interpret the phrase "record amount" as ambiguous, because I can't recall an instance anywhere where I've read the phrase "record amount" (without a high or low qualifier) to mean "record low amount." Instead it always seems to mean "record high amount." Now that doesn't mean the phrase absolutely couldn't be read as ambiguous. And I suppose there's some chance the LSAT writers intend you to spot that ambiguity. But I would generally recommend reading a phrase like that the way it (uniformly?) seems to be used, which is as "record high amount" (even though it's true the sentence doesn't explicitly say that!).

Second, with Parallel questions, I'd also recommend you always look at the bigger picture as well. What's the overall abstract description we can give to the questionable argument in the stimulus? James does a great job above of labeling it a Mistaken Reversal (which is questionable reasoning). Does answer choice B fit that description? It doesn't, for the reasons James lays out so well above, so that's a more secure reason to eliminate it from contention.

I hope this helps!