Necessary assumptions and the Negation Test
Find the gap first.
Test each answer choice by negating it. Plug the negation back into the argument: if the argument now falls apart, that choice was a necessary assumption. If the argument survives the negation unharmed, that choice was never actually required.
Sufficient assumptions close the whole gap
A sufficient assumption question asks for more: an answer that, if simply added as a premise, would make the conclusion follow with certainty, no further help needed. Every necessary assumption question also implies the argument needs something; not every sufficient assumption is required — a stronger, less obviously connected answer can still work if it seals the gap completely.
Read the question stem closely. Necessary-assumption stems typically say the argument 'depends on' or 'requires' an assumption; sufficient-assumption stems typically say the assumption 'would allow the conclusion to be properly drawn' or similar guarantee language.
What LSAC says about detecting assumptions
LSAC includes detecting assumptions made by particular arguments in its list of the skills Logical Reasoning tests. It also says these questions rest on short arguments from newspapers, magazines, scholarly publications, advertisements and informal discourse, so an assumption question is usually about an ordinary-language argument rather than a formal proof.
LSAC adds that you should have a university-level understanding of concepts such as argument, premise, assumption and conclusion, and it points to its SuperPrep and SuperPrep II books for explanations of how those concepts are used in Logical Reasoning questions.
3 original practice questions
Answer each question, then check it. Your result is saved on this device and counts toward your study dashboard.
Question 1
City officials argue that because traffic fatalities decreased by 12% in the year after new speed cameras were installed, the cameras caused the decrease. Which of the following is an assumption required by the officials' argument?
Question 2
A company promotes only employees who have completed its leadership seminar. Maria has completed the leadership seminar. Which of the following, if assumed, guarantees that Maria will be promoted?
Question 3
A nutritionist claims that eating breakfast every day improves cognitive performance, citing a study in which students who ate breakfast scored higher on memory tests than students who skipped it. The argument assumes that
Common questions
What is an LSAT assumption question?+
It asks what an argument requires to be true. A necessary assumption is something the argument cannot work without, while a sufficient assumption would fully guarantee the conclusion if added.
What is the Negation Test on the LSAT?+
Negate an answer choice and plug it back into the argument. If the argument then falls apart, the choice was a necessary assumption, and if the argument survives, the choice was never required.
How do necessary and sufficient assumptions differ?+
A sufficient assumption is stronger: added as a premise, it would make the conclusion follow with certainty, while a necessary assumption is only something the argument cannot do without.
How can I tell a necessary-assumption stem from a sufficient-assumption stem?+
Necessary stems typically say the argument depends on or requires an assumption, while sufficient stems typically say the assumption would allow the conclusion to be properly drawn.
Where is the gap in an LSAT assumption argument?+
Often a causal claim resting on mere correlation, or a claim about one group applied to a different group. Find what the conclusion needs that the stated premises do not supply.
Should I read the answer choices before finding the conclusion?+
No. Identify the argument's conclusion and premises first, then test the choices, because the gap you find determines which choice can be an assumption.
What does LSAC say assumption questions test?+
LSAC lists detecting assumptions made by particular arguments among the skills tested in Logical Reasoning, using short arguments from sources such as newspapers and magazines.
Where does LSAC explain premise and assumption?+
In its SuperPrep and SuperPrep II books, which LSAC says explain concepts such as argument, premise, assumption and conclusion and how Logical Reasoning questions use them.
What to do next
- 1
Identify the argument's conclusion and premises before reading the choices.
- 2
Apply the Negation Test to any choice that looks close on a necessary-assumption question.
- 3
On sufficient-assumption questions, check whether the choice, once assumed, leaves zero room for the conclusion to fail.
- 4
Work all three practice questions below before checking the explanations.