Strengthen vs. Sufficient Assumption on the LSAT: How to Tell Them Apart

Strengthen questions and sufficient assumption (SA) questions both look like they're asking you to 'help' an argument. In both cases, you're looking for an answer that benefits the conclusion. Students learning these types often treat them as essentially the same question with different wording.

They're not. The difference is in how much work the correct answer has to do. A strengthen answer only needs to make the conclusion more likely — it nudges the argument toward being right. A sufficient assumption answer needs to make the conclusion follow with certainty — it has to close the logical gap completely so that if the premises and the SA are true, the conclusion *must* be true.

A strengthen answer raises the probability of the conclusion being true, given the premises. It doesn't have to guarantee anything. An answer that provides partial support, rules out one alternative explanation, or adds a relevant data point is enough — as long as it makes the conclusion more defensible.

A sufficient assumption answer, when added to the premises, makes the argument logically valid. The conclusion no longer has any daylight — it follows necessarily from the premises plus the SA. If the answer doesn't fully close the gap, it fails as an SA answer even if it would be a fine strengthen answer.

In formal terms: strengthen → the argument's conclusion becomes more probable. Sufficient assumption → the argument becomes deductively valid (the conclusion cannot be false if the premises and SA are true).

Strengthen stems: 'Which of the following, if true, most strengthens the argument?', 'Which of the following provides the most support for the conclusion?'

Sufficient assumption stems: 'The conclusion follows logically if which of the following is assumed?', 'Which of the following, if assumed, allows the conclusion to be properly drawn?', 'The argument's conclusion can be properly inferred if which one of the following is assumed to be true?'

The language of SA stems is logically demanding: 'follows logically,' 'can be properly drawn,' 'properly inferred.' These phrases signal that the answer must make the conclusion *necessary*, not merely more plausible.

Here's the trap: a strengthen answer is almost always a tempting wrong answer on an SA question. It genuinely helps the argument — it just doesn't help it enough.

Suppose an argument concludes: 'This company's new drug is safe for human use.' Premise: 'The drug passed all required animal trials.' A strengthen answer might be: 'The drug has no known interactions with common medications.' That's good supporting evidence. But it doesn't make 'safe for human use' logically guaranteed from the animal trial premise alone.

A true SA answer would need to close the specific inferential gap the argument relies on. Something like: 'Any drug that passes all required animal trials is safe for human use.' Now the conclusion follows necessarily — if the premise is true and the SA is true, the conclusion cannot be false.

Notice the strength of language in that SA: 'any drug.' It's sweeping. SA correct answers are often surprisingly strong or absolute-sounding because they need to fully bridge the gap.

The reverse trap also exists: a logically airtight SA answer can be wrong on a strengthen question because it's overkill. SA answers often introduce universal claims or categorical rules — 'All X are Y' — that go further than the argument needs.

On a strengthen question, extreme or universal answer choices should make you cautious. Strengthen answers tend to be more measured: they support without overreaching. An answer that says 'every drug with this molecular structure is completely safe for all human populations' sounds like it helps, but its absolute nature can make it less believable or more likely to introduce other problems.

This matters less mechanically (the LSAT does sometimes use strong language on strengthen answers), but it's a useful heuristic: if an answer is strikingly absolute, check whether you're on a strengthen or an SA question.

After reading the question stem, ask yourself: does this question need an answer that *guarantees* the conclusion, or just one that *supports* it?

If the stem uses 'follows logically,' 'properly drawn,' 'must be assumed,' or similar language — you need a guarantee. Filter for the answer that, combined with the premises, makes the conclusion airtight.

If the stem says 'most strengthens,' 'provides support,' or 'most helps' — you need support, not a guarantee. Pick the answer that best raises the probability of the conclusion being true.

For SA questions specifically, the negation test is a useful check after you've chosen an answer: negate the answer choice. If negating it collapses the argument (makes the conclusion fail), you've found the sufficient assumption. If negating it merely weakens the argument a little, the answer is a strengthen-quality choice, not an SA.