Logical reasoning PrepTest 146 · Section 1 · Question 9

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Study Ð More plant species in a given plot was correlated with better growth and soil
Conclusion Ð Having more plant species improves a plot

Answer Anticipation:
Whenever a study is done in a question with a flawed argument, you should start by considering two flaws—sampling, and correlation/causation.

Here, the sample is specified outside of "prairie plants/plots," but the conclusion is about this group, so there's no hint of a sampling flaw.

However, there are signs of a correlation/causation flaw. First, the premise establishes a non-causal relationship—the more X, the more Y. That's a correlation. And the conclusion is causal ("improves"). Since this argument jumps from a correlation to causation, the correct answer should highlight that flaw. And since reverse causality makes sense here—good soil might result in more species of plants being able to thrive—that's likely to feature in the correct answer.

Answer Explanation:
This answer highlights the correlation/causation flaw. It even throws in a nod to reversed causality since that makes sense in this scenario. This is our answer!

Key Takeaway:
First, knowing the way that common flaws are described in answers is very helpful in quickly understanding and eliminating wrong ones. Here, (A), (D), and (E) were all answers that have shown up many times on the LSAT to describe specific flaws (correlation/causation; samplines; percent and amount).

Second, there are some repeated answers that describe things that aren't flaws that show up repeatedly on the LSAT. (B) and (C) are both describing things that aren't inherently flawed and thus will rarely (if ever) be correct, even though the LSAT will offer them repeatedly.

Incorrect. A causal relationship can be validly established without defining the exact mechanism by which it works, so this answer will rarely if ever be correct.

Incorrect. First, the stimulus must have studied multiple plots since it's able to compare ones with more and fewer plant species. Second, the whole idea of running a study is that looking at one example can tell you something about others, so it's not even clear this is a flaw. If it were a flaw to assume this, then any sample would be suspect.

Incorrect. This is a more traditional sampling flaw answer, but there's no indication that the sample here is likely to be unrepresentative—the study looked at prairie plants in prairie plots, and it reached a conclusion on the same subjects.

Incorrect. Both the premises and conclusion are about an increase in the number of plant species, so there's no jump between that and proportion.