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

[MillsV]

Hi, for this one I chose A. I saw that the stimulus is a mistaken reversal and A was a mistaken negation, but I thought that since those two mistakes are contrapositives of each other like it states in the powerscore books, that A showed a parallel to the stimulus. So, will a contrapositive of the correct parallel flaw always be incorrect?

(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.

For E, I have a hard time drawing the diagram. To me, the premise can be drawn as Detection—> outbreak, and the conclusion has the same diagram. I do not see the sufficient or necessary indicator in the premise. Could you please explain why the diagram is the other way around?

[KelseyWoods]

HI MillsV!

You are correct that the Mistaken Reversal and Mistaken Negation are contrapositives of each other and so are logical equivalents. But in a Parallel question, we really want the argument to match the structure as much as possible. That means that if you have a Mistaken Reversal in the stimulus, you want a Mistaken Reversal in the answer choice and if you have a Mistaken Negation in the stimulus, you want a Mistaken Negation in the answer choice. This is because even though the underlying logic is flawed in a similar way, the author is using different evidence to prove a different conclusion. In the Mistaken Reversal, the author is taking evidence that the necessary occurs to conclude that the sufficient occurs. In the Mistaken Negation, the author is taking evidence that the sufficient condition did not occur to conclude that the necessary condition did not occur. Yes, in both of these the author is mistaking the sufficient for the necessary condition. But the way in which the evidence is used to support the conclusion is different. In Parallel questions, we need the evidence to support the conclusion in the same way.

For answer choice (E), "all" is a sufficient condition indicator. In the first sentence, "all" modifies "outbreaks" so you would diagram the first sentence as: outbreaks detection

Hope this helps!

Best,
Kelsey

[MillsV]

Thank you so much Kelsey!

[sicm91]

Hi! I may be overthinking this, but isn't saying that the river will overflow an incorrect assumption since perhaps this year may be different? In other words, just because it happened before doesn't mean it will happen again?

[Robert Carroll]

sicm,

Yes, that's correct; answer choice (B) is flawed. Because it's not flawed the same way as the stimulus, it's incorrect.

Robert Carroll

[lounalola]

Why are A and C incorrect for this?

[Luke Haqq]

Hi lounalola!

In the stimulus, the flaw is that the author reasons that phenomenon A (earthquake) has always been preceded by phenomenon B (tremors), so the author concludes that because phenomenon B exists, then it must be followed by phenomenon A. However, that doesn't follow. Just because each earthquake is preceded by tremors, it doesn't mean that all tremors are followed by earthquakes. There could be tremors followed by no earthquake.

Regarding answer choice (A), it seems flawed, but it's not the same type of flaw as in the stimulus. It's not reasoning that phenomenon A has always been preceded by phenomenon B, with the author concluding that because phenomenon B exists, then it must be followed by phenomenon A. Claire's post above provides a good summary of the flaw in (A).

Answer choice (C) also isn't following that same type of flawed reasoning. Answer choice (C) is providing the following conditional reasoning:

RM HMB

We're then told that RM are present on Earth, which seems to be reason to conclude that there must have been HMB. I think a potential issue with this answer choice is that we're only told about planets other than Earth. What if these same minerals were found on Earth but it differed from other planets in that they weren't there as a result of HMB? More to the point, again, this doesn't reflect the same type of flaw as in the stimulus.