Wednesday, March 7, 2012

Logical Reasoning: Most Logically Completes

Today we'll explain how we approach one of the tasks, most logically completes, which is where the LSAT asks you to identify the answer that provides the argument's missing structural or evidentiary element.

This task is prompted by a predictable question stem, and is possibly the easiest task of all to identify because of the "blank" space at the end of the stimulus, i.e. "because _______." On modern LSATs, the most common version of this question stem is:
Which one of the following most logically completes the argument?
Unfortunately, most logically completes and Cards Against Humanity only share the _________ and the English language in common.
This task benefits the most from the extrapolation strand's basic visualization:

Two general structures that Zen of 180 students use to visually represent the must also be true task

In the example on the left, the credited answer (C), is a necessary condition interpolated between two or more pieces of evidence presented in the stimulus (A and B), i.e. in order for B to be true in the context of A, C must also be true.  In the diagram on the right, the credited response is a combination or two or more pieces of evidence from the stimulus, that when combined lead to a new conclusion.

The key to correctly answering a most logically completes question is to clearly highlight the main pieces of evidence--the actors and their definitions--and how they have been  combined in terms of relationships and degree of certainty, paying special attention to opinions.  As with other extrapolation standards, the goal is not to prephrase the specific evidence--the LSAT answers will invariably mention the "correct" parts--but rather the links in relationship and the degree of certainty between the pieces of the stimulus.

However, because the stimulus is missing the text in the correct answer choice, it can also be helpful to classify the argument's pieces (premise, evidence, bridge, and conclusion). This is because if the stimulus does not have a conclusion, then the correct answer choice will be the conclusion; however, if the stimulus ends with "because __________.", then the answer will provide supporting evidence or the bridge to existing evidence.

In this example, actors are black, relationships are red, and degrees of certainty are cyan.
The above example shows how the answer choice focuses on explaining the key discrepancy set up in the argument, why nettle plants should be used even though they attract aphids; the stimulus' degree of certainty that aphids were not uniformly bad left open the possibility that there could be some that do not eat the potato plants. The correct answer choice helpfully shows that the nettle plants only attract these benign aphids.