Key takeaways
- A sufficient assumption, added to the premises, guarantees the conclusion.
- Find the term that appears in the conclusion but not the premises, then link it to the evidence.
- Sufficient answers can be stronger than needed. That's fine. Weak answers are the problem.
- Necessary answers are required but may not be enough. The Negation Test checks necessary answers, not sufficient ones.
Sufficient assumption questions ask for the piece that makes an argument work completely. They look like necessary assumption questions, and students often mix them up, which costs points on both types. This guide shows the bridge method I teach, how to tell sufficient from necessary, and an original question to try.
How to spot a sufficient assumption question
The stem asks for something that, if assumed, would make the conclusion follow. Look for wording like:
- "The conclusion follows logically if which one of the following is assumed?"
- "Which one of the following, if assumed, enables the conclusion to be properly drawn?"
- "…allows the conclusion to be properly inferred?"
The key words are "if assumed," "follows logically" and "properly drawn." Compare that with necessary assumption stems, which say the argument "requires," "depends on" or "relies on" something.
Sufficient vs. necessary: the one-sentence difference
- A sufficient assumption is enough: add it and the conclusion is guaranteed.
- A necessary assumption is required: without it the argument falls apart, but adding it might not be enough.
Picture a gap between two cliffs. A necessary assumption is one plank the bridge can't do without. A sufficient assumption is a whole bridge. It might even be wider than it needs to be.
That's why a sufficient answer can say "all," "every" or "always" and still be right. Strong language that would sink a necessary assumption answer is often exactly what a sufficient answer needs. If you want the other half of this, read the necessary assumption guide.
The bridge method
Step 1: Find the conclusion and the premises. Write the conclusion in a few words.
Step 2: Find the new term. Look for an idea that appears in the conclusion but nowhere in the premises. That's what the argument hasn't earned yet. Often there's also a premise term that never reaches the conclusion.
Step 3: Build the bridge. Predict a statement that links the premise term to the new term: "If [premise idea], then [conclusion idea]." Usually that's the answer, or something equivalent to it.
Step 4: Check it. Add your answer to the premises. Does the conclusion now follow with no doubt left? If there's still a "maybe," the answer isn't sufficient.
Many sufficient assumption questions are built on conditional logic, so translation skills pay off here. If "unless" or "only if" slows you down, start with our guide to those two words.
Sample question (LSAT JET original)
Every member of the Hartwell College budget committee is a tenured professor. So every member of the budget committee is eligible to vote in the faculty senate.
The conclusion follows logically if which one of the following is assumed?
(A) Every professor eligible to vote in the faculty senate is tenured. (B) Some tenured professors serve on the budget committee. (C) Every tenured professor at Hartwell College is eligible to vote in the faculty senate. (D) Most members of the faculty senate serve on at least one committee. (E) No professor who is not tenured serves on the budget committee.
Answer: (C).
- Premise: Committee → Tenured.
- Conclusion: Committee → Eligible to vote.
- New term: "eligible to vote." The premise term that hasn't reached the conclusion is "tenured."
- Bridge: Tenured → Eligible. That's (C).
Check it: Committee → Tenured → Eligible. The conclusion is guaranteed.
- (A) reverses the bridge: Eligible → Tenured. It doesn't get a committee member to "eligible."
- (B) "Some" can't guarantee anything about "every" member.
- (D) is off topic.
- (E) just restates the premise in contrapositive form. It adds nothing new.
Common traps
The reversed bridge. Answer (A) above is the classic sufficient assumption trap: the right two terms, linked the wrong way. Always check the arrow direction against what the conclusion needs.
The necessary-but-not-enough answer. Some wrong answers are things the argument does need, but they don't finish the job. In sufficient questions, "needed" isn't the test. "Enough" is.
The restatement. An answer that repeats a premise, even in different words, can't be sufficient. It doesn't touch the new term.
Using the Negation Test here. The Negation Test (negate the answer and see if the argument collapses) is built for necessary assumptions. Don't use it on sufficient ones. A right sufficient answer can fail the Negation Test because it's stronger than the argument needs.
When the bridge has two planks
Some arguments have two gaps, or a premise term and a conclusion term that need an extra step to connect. Don't panic. Write the premises as a chain and see where it breaks. The right answer fills the break, and sometimes it does so by linking a premise to the conclusion in contrapositive form. If your predicted bridge is "Tenured → Eligible" and an answer says "No one who is ineligible is tenured," that's the same bridge, just flipped. Check each answer's arrow, not just its words.
Sufficient vs. necessary at a glance
| Sufficient assumption | Necessary assumption | |
|---|---|---|
| Stem words | "if assumed," "follows logically," "properly drawn" | "requires," "depends on," "relies on" |
| What it does | Guarantees the conclusion | Must be true for the argument to work |
| Strong words (all, every) | Often fine | Often a trap |
| Best check | Add it: is the conclusion now certain? | Negate it: does the argument fall apart? |
How to practice
- Collect ten sufficient assumption questions from your practice tests.
- Before reading the answers, write the new term and your predicted bridge.
- After answering, check: did the right answer match your bridge (or its contrapositive)?
- In blind review, log every miss as "reversed," "not enough" or "restatement." One of those will dominate, and that's your fix.
Frequently asked questions
What is a sufficient assumption on the LSAT?
It's a statement that, if added to the argument's premises, guarantees the conclusion. The stem usually says the conclusion "follows logically" or can be "properly drawn" if the answer is assumed.
What is the difference between a necessary and a sufficient assumption?
A necessary assumption must be true for the argument to work, but it may not be enough. A sufficient assumption is enough to prove the conclusion, but it may be stronger than the argument needs.
Can a sufficient assumption answer use strong words like "all"?
Yes. Strong language is often exactly what makes an answer sufficient. Extreme wording is a common trap in necessary assumption questions, not in sufficient ones.
Does the Negation Test work on sufficient assumption questions?
No. The Negation Test checks whether an answer is necessary. A correct sufficient answer can fail it because it says more than the argument requires.
Next step
Small live classes, capped at 12 students. Alan teaches every session himself and reviews your mistakes.
Assumptions are session 3 of both January tracks. Book a free 15-minute course call and we'll talk through which track fits you.
Related articles
- "Unless" and "Only If" on the LSAT: How to Translate Them
Two words cause more conditional-logic mistakes than any others. Here are the two rules that fix them.
- What "Takes for Granted" Means on the LSAT
It means "assumes without support." Here is how to test that kind of answer in two checks instead of guessing.