Recognizing common argument gaps is one of the most critical skills for the LSAT and logical reasoning in general. A "gap" is the unstated assumption the author relies on to make the conclusion follow from the premises.

There are 5 most common structural gaps.

1. The Sampling Gap (Part vs. Whole)

The Pattern: The author studies a small group and makes a claim about a large group.
The Gap: The sample is not representative of the whole population.

How to spot it: Ask: Is the thing they studied actually like the thing they are talking about?

• Example: "Our online survey of 500 readers shows 90% prefer e-books. Therefore, the general population prefers e-books."
• Gap: Online readers are not the general population.

2. The Correlation vs. Causation Gap

The Pattern: Two things happen at the same time (or one happens after the other), so the author concludes one caused the other.
The Gap: There is a third variable, or the reverse causation is true.

How to spot it: Ask: Could something else have caused this? Did A actually cause B, or did B cause A?

• Example: "Since ice cream sales increase in the summer and drownings increase in the summer, ice cream sales cause drowning."
• Gap: Hot weather (third variable) causes both.

3. The Equivocation Gap (Apples to Oranges)

The Pattern: The argument uses the same word in two different ways.
The Gap: The meaning of a key term shifts between the premise and the conclusion.

How to spot it: Look at the definition of the key noun in the evidence, and then look at the definition in the conclusion. Are they the same thing?

• Example: "The end of a matter is its completion. Therefore, death (the end of life) is the completion of life."
• Gap: "End" as in "goal/purpose" vs. "End" as in "termination."

4. The "Other Possibilities" Gap (False Dilemma)

The Pattern: The author presents two options and argues that since one is bad/untrue, the other must be true.

The Gap: There are more than two options.

How to spot it: Ask: Are these the only two possibilities in the universe?

• Example: "You are either with us, or you are against us. You are not with us, so you are against us."
• Gap: Neutrality or indifference.

5. The Sufficiency vs. Necessity Gap (Conditional Logic)

The Pattern: The argument confuses the direction of a conditional statement.
The Gap: Mistaking "If A then B" for "If B then A."

How to spot it: Diagram the logic.

• Example: "To be a senator, you must be 30 years old. John is 30, so he is a senator."
• Gap: Being 30 is necessary for being a senator, but it is not sufficient. There are other requirements (e.g., being elected).