Today we'll explain how we approach one of the tasks, must also be true, which is where the LSAT asks you to extrapolate a new statement using formal logic based on information presented in a stimulus.
This task is prompted by a fairly pedestrian question stem, as exemplified by the explanations for the October 1996 Sample PrepTest below. On modern LSATs, the most common version of this question stems is:
If the [stimulus] above is true, which one of the following must also be true?
|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 must also be true question is to clearly highlight the main pieces of evidence--the actors and their definitions--and how they can be combined in terms of topic, degree, certainty, and opinion. As with depends upon assuming, the goal is not to describe the specific evidence--as 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.
Above is a gallery of the logical reasoning sections that have a greater than average density of the task, and below is the frequency that this task has been asked on modern LSATs and the percentage change in frequency from pre-2007 LSATs. It averages 2.4% of the points on a modern LSAT, and you can reasonably expect that one or two points will be devoted to making concrete logical extrapolations.
on LSATs since 2007
|2.4% of LR|
(~1 per LSAT, range 0-2)
|-1.3% growth from pre-2007|