Logical Reasoning: Parallel and Parallel Flaw Questions

Parallel Reasoning questions in the LSAT's Logical Reasoning section don't always mention that there's a flaw in the stimulus.

In other words, not every Parallel Flawed Reasoning question is described as such in the question stem - some simply present themselves as general Parallel Reasoning questions.

However, knowing there is a flaw in the stimulus, even if you have to identify it for yourself, is useful in solving this type of question, as it gets to the core issue of the argument's method of reasoning.

For example, take this LSAT Logical Reasoning question stem and the question itself:

(It's PrepTest 31 (June 2000 LSAT), Section 3, Question 18 - page 100 in Next 10)

"The pattern of reasoning in which one of the following is most similar to that in the argument above?"

There's no mention of anything wrong with the method of reasoning in the stimulus.

If we were to take the claims of some LSAT prep books as true, then we'd falsely assume that the stimulus' method of reasoning is fine, making it harder to solve the question.

However, the question stem of the Logical Reasoning question on page 100 in Next 10 that I just mentioned does not contain any reference to flawed or questionable reasoning. It contains a major flaw.

How could this be?

Well, the LSAT is not obligated to tell you when a stimulus contains flawed reasoning. They often do, but this doesn't mean that they have to.

Whether a method of reasoning is said to be flawed or not, one can still technically find reasoning that is similar to it.

As such, the LSAT creators aren't doing anything wrong by not explicitly stating there's flawed reasoning - they're just making it a bit harder to solve the question by making you recognize the flawed reasoning on your own.

So, how is PrepTest 31 (June 2000 LSAT), Section 3, Question 18 exhibiting flawed reasoning?

To sum it up, the stimulus claims (the following is my interpretation - LSAC does not allow me to publish LSAT questions on the blog itself due to copyright issues):

science requires measuring, and measuring requires units of measurement. Because the unit of measurement that ones uses is arbitrary, therefore, science itself is arbitrary.

However, just because one aspect of science is arbitrary, this doesn't guarantee that science as a whole is arbitrary. It's possible that other aspects of science outweigh the arbitrariness of the unit of measurement selected.

The flaw is like saying that just because carrot cake might include some salt, therefore carrot cake as a whole is salty.

This is often called a part-to-whole flaw. Just because part of something has a certain characteristic, this doesn't mean the entire thing has that particular characteristic.

The correct answer choice exhibits the same flawed method of reasoning as the stimulus - it takes one aspect of a particular pursuit and incorrectly assumes that the pursuit as a whole can then be said to feature that same characteristic.


The question from PrepTest 31 that I describe above is not the only case where a Parallel Flawed Reasoning question does not explicitly state in the question stem that the argument contains flawed reasoning.

Another Example: PrepTest 44, Section 4, Question 21

Its question stem states:

"Which one of the following arguments employs reasoning most similar to that employed by the argument above?"

The argument assumes that it is *better* to drive a small car than a large one because smaller ones avoid more accidents in the first place, as they are easier to drive. However, smaller cars don't offer as much physical protection as larger ones, they are easier to drive.

As such, the argument engages in incomplete and questionable reasoning. It fails to consider that perhaps physical protection when one DOES get into an accident is more important than ease of driving. It never explicitly gives a reason why one characteristic should be valued over the other one. The argument would be stronger if it cited as evidence a scientific study or general principle giving support to the value of accident avoidance over protection from accidents that do occur.

Yet Another Example:
PrepTest 48, Section 1, Question 12

Its question stem states:

"The reasoning in the argument above is most closely paralleled by the argument that there is no reason to"

This argument fails to consider that there may still be some legitimate reason or "justification" for delaying the process by which species become extinct even if they will eventually become extinct regardless.

It's like saying that there's no reason to try and keep human beings alive by feeding them or giving them medical care since they will inevitably die regardless, since we're all mortal.


The takeaway:

Be aware that even if a Parallel question does not mention that the reasoning contained in the stimulus contains flawed or questionable reasoning, it may do so anyway. Recognizing the flaw in the stimulus is key to recognizing the answer choice containing the reasoning that is most similar.


  1. Good point that I was not aware of earlier. Will watch out for this now.

  2. Steve, you the man!

  3. i hate theses questions so bleeding much...

  4. This seems to contradict Chapter 14 of the LRB:

    (p. 401)
    "Since the February 1992 LSAT, whenever a Parallel Reasoning question contains flawed reasoning, it is stated in the question stem. If there is no mention of the flawed reasoning in the question stem, the reasoning in the stimulus is valid (and vice versa)."

    Could you clarify this for me?

    1. Sure. The quoted text is incorrect. This is an example of what I was alluding to above.

  5. Steve,

    "science requires measuring, and measuring requires units of measurement. Because the unit of measurement that ones uses is arbitrary, therefore, science itself is arbitrary."

    If I write the above in a diagram, it would be:

    science -> measuring -> units of measurement,

    And according to your post, "Logical Reasoning: Necessary and Sufficient Conditions," the sufficient condition is a specific example/subset of the necessary condition. According to this logic, measuring is a subset of units of measurement, and science is a subset of measuring.

    However, in this post, units of measurement is an aspect of science, not the other way around. Could you clarify this for me?

    1. Good question, let me take a jab at this.
      Saying units of measurement are arbitrary (modifier) is different from saying units of measurement don't exist/not possible to determine (negation), therefore the contrapositive thing doesn't work here and it doesn't loop back to science. Therefore, though units of measurement (an aspect of science) in science might be arbitrary, to go from that to science is arbitrary would require a brand new assumption for it to work.

  6. This comment has been removed by the author.

  7. Great as always, Steve! Thank you, thank you! :)