Assumption questions ask for the unstated premise an argument depends on. They come in two distinct flavors that reward different moves: necessary assumption questions ("the argument depends on which of the following?") ask for something the argument can't survive without, while sufficient assumption questions ("which of the following, if assumed, allows the conclusion to be properly drawn?") ask for a premise strong enough to guarantee the conclusion.
Confusing the two is one of the most common, and most fixable, sources of lost LR points. A necessary assumption can be a small, almost timid claim; it just has to be indispensable. A sufficient assumption is usually a big, sweeping claim; it has to close the gap completely. The same answer choice can be right for one task and wrong for the other.
Together the two subtypes account for roughly four to seven questions per test, and the underlying skill, seeing the gap between evidence and conclusion, is the same one flaw, strengthen, and weaken questions are built on. Assumption training is really gap-spotting training.
Stem language that signals this type:
"Required," "depends," "relies" → necessary. "If assumed, the conclusion follows" → sufficient. Decide before reading choices, because the right-answer profile is opposite: modest for necessary, strong for sufficient.
Negate the answer choice. If the negated version wrecks the argument, the original was necessary. If the argument limps along fine without it, it wasn't required. The negation test is the single most reliable tiebreaker on the test, use it on your final two.
Find the concept that appears in the conclusion but never in the evidence. The right answer connects the evidence's terms to that new concept, usually in the form "if [evidence], then [conclusion]." Treat it like completing a proof.
On necessary questions especially, an answer can strengthen the argument enormously and still not be required. "Helpful" is the wrong standard. Ask: could the argument survive without this?
An original question written in the LSAT style, not from a real test.
City planner: Installing dedicated bus lanes on Harden Avenue will reduce commute times along that corridor. Buses currently carry forty percent of Harden Avenue commuters, and with a dedicated lane, buses will move significantly faster during peak hours.
The planner's argument depends on assuming which one of the following?
Pick an answer to check your reasoning.
"All," "only," "never" answers are usually too strong to be required, the argument typically needs something weaker. Reserve strong answers for sufficient stems.
The mirror image: a "some" or "might" answer almost never guarantees a conclusion. If it leaves any daylight, it isn't sufficient.
An assumption is unstated by definition. An answer that paraphrases something already in the stimulus can't be the assumption, no matter how central it sounds.
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A necessary assumption is something the argument requires, remove it and the argument fails. A sufficient assumption is something that, if granted, makes the conclusion follow completely. Necessary answers tend to be modest; sufficient answers tend to be strong bridges between evidence and conclusion.
Logically negate an answer choice on a necessary assumption question. If the negation destroys the argument, that choice is required and therefore correct. Negate carefully: the negation of "all X are Y" is "at least one X is not Y," not "no X are Y."
Yes, but they share a core: both hinge on the gap between evidence and conclusion. Many students drill them together as a "gap family," then add the type-specific tools, the negation test for necessary, term-bridging for sufficient.
Usually four to seven combined across both scored LR sections, typically more necessary than sufficient.
Name the specific error in the argument's reasoning.
Pick the new fact that makes the conclusion more believable.
Bridge general rules and specific cases, in both directions.
Or head back to all question types · What is the LSAT?
Last updated July 9, 2026 · by the theLSATguru team