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I am told that for a parallel reasoning question, the question stem will aways tell whether the reasoning in the stimulus is sound or flawed. That if the question stem does not mention flaws, then I am to look for an answer choice that has sound logic.

But for PT 4 LR 2 Q 20, which is a parallel reasoning question, the question stem does not say anything about flawed reasoning yet it is a flawed parallel reasoning question.

I used to assume that the reasoning in a parallel question is sound unless the question stem indicates otherwise. But I am starting to question this rule.

Has anyone encountered similar experiences?
Is it because it is PT 4 and that the aforementioned rule only applies to more recent LSATs?

Can someone confirm my suspicion?

The stem will usually tell you the reasoning is flawed when it is, but not always. It doesn't occur very often and questions of the sort seem to have appeared more frequently in old tests than in relatively recent ones. Don't know offhand the most recent test in which one occurred, but it was within the last five years if my memory serves correctly, so don't assume that the argument will not be flawed in a parallel question on a new test that doesn't say the reasoning is flawed, it could be. Luckily questions of this sort are rare so assuming the argument is not flawed usually will be fine, but you could get unlucky with that strategy if one appears on your test.

Since either way your job is to determine the method of reasoning of the argument in the stimulus, if the reasoning is flawed you should figure that out anyway whether or not you assumed it was not flawed as long as you analyze the reasoning carefully and are familiar with the common flawed methods of reasoning that reoccur on the LSAT.

Thanks for confirming it.

I have a follow up:
If someone says X happened only when Y was adopted. Is is correct to diagram that as X-->Y?
Manhattan LSAT says that I shouldn't diagram that as a conditional. But according to Testmasters "only when" is the temporal version of "only if" and thus the two phrases both indicate necessary conditions.

Based on PT 4 LR 2 Q 20, who is right? (The stimulus says, "Economic conditions improved only when Policy X had been adopted.")

If Manhattan were correct, then when should I interpret "only when" as indicating a necessary condition?
I never had a problem interpreting "only when" until this question. The older tests really are shaking things up for me.

TTX wrote:I am told that for a parallel reasoning question, the question stem will aways tell whether the reasoning in the stimulus is sound or flawed. That if the question stem does not mention flaws, then I am to look for an answer choice that has sound logic.

But for PT 4 LR 2 Q 20, which is a parallel reasoning question, the question stem does not say anything about flawed reasoning yet it is a flawed parallel reasoning question.

I used to assume that the reasoning in a parallel question is sound unless the question stem indicates otherwise. But I am starting to question this rule.

Has anyone encountered similar experiences?
Is it because it is PT 4 and that the aforementioned rule only applies to more recent LSATs?

Can someone confirm my suspicion?

This is a common misconception. Here are some other examples in more recent LSATs:

http://lsatblog.blogspot.com/2011/04/lo ... -flaw.html


Re: your other question, "only when" = "only if"

"X only when Y" can be diagrammed as "X --> Y"

"X only if Y" can be diagrammed as "X --> Y"

LSAT Blog wrote:

This is a common misconception. Here are some other examples in more recent LSATs:

http://lsatblog.blogspot.com/2011/04/lo ... -flaw.html


Re: your other question, "only when" = "only if"

"X only when Y" can be diagrammed as "X --> Y"

"X only if Y" can be diagrammed as "X --> Y"

But when I conclude from "X only when Y" that Y caused X, would I be making a temporal flaw? or an incorrect reversal? or both?

Thank you for the article and all the other articles you keep on your blog

TTX wrote:

LSAT Blog wrote:

This is a common misconception. Here are some other examples in more recent LSATs:

http://lsatblog.blogspot.com/2011/04/lo ... -flaw.html


Re: your other question, "only when" = "only if"

"X only when Y" can be diagrammed as "X --> Y"

"X only if Y" can be diagrammed as "X --> Y"

But when I conclude from "X only when Y" that Y caused X, would I be making a temporal flaw? or an incorrect reversal? or both?

Thank you for the article and all the other articles you keep on your blog

Glad you enjoy the blog!

Re: temporal flaw, let's look at an example:

Original: I'm full only when I've eaten. (F --> E)

Converse: I eat only when I'm full. (E --> F)

In the original, eating occurs before fullness.

In the converse, fullness occurs before eating.

So, yes, reversing the conditions (the converse flaw) when dealing with "only when" statements would be a temporal flaw.

Given X --> Y, inferring that Y requires X would be a mistake - the flaw of the converse (aka incorrect or mistaken reversal).