Critical Reasoning
Verbal
7 min readAssumption questions ask what the argument must take for granted. The correct answer is not a fact the author states. It is a gap the author leaves open, a belief that has to be true for the evidence to support the conclusion. Most wrong answers are plausible ideas that the argument could survive without.
An assumption is an unstated premise. It connects the evidence to the conclusion. If the evidence is "the new software reduced errors by 30 percent" and the conclusion is "the company should buy the software," the argument assumes that fewer errors are worth the cost, that the software will work in the company's setting, and that no other factor explains the drop. The correct answer will identify one of these gaps.
Not every unstated idea is an assumption. The argument can survive without most background facts. The test is whether the conclusion still holds if the idea is false. If the conclusion collapses, the idea is a necessary assumption.
An assumption is necessary, not sufficient. It does not have to prove the conclusion. It only has to be something the argument cannot do without.
The most reliable way to test an assumption answer is to negate it. If the negated statement destroys the argument, the original statement is a necessary assumption. If the argument still works, the answer is wrong.
Example. Argument: The city should add more bike lanes because bike commuting reduces traffic congestion.
Possible assumption: Bike commuters will use the new lanes instead of driving.
Negation: Bike commuters will not use the new lanes instead of driving.
If commuters do not switch from cars to bikes, the lanes do not reduce congestion. The argument fails. The assumption is necessary.
Negate carefully. "Some" becomes "none." "All" becomes "not all," which means at least one does not. "At least one" becomes "none." A sloppy negation produces a false result.
Most GMAT arguments have three parts:
Find the conclusion first. It is often introduced by words such as "therefore," "thus," "so," "clearly," or "the plan will." The evidence usually follows "because," "since," or "studies show." Once you have both, ask what must be true for the evidence to support the conclusion.
Before reading the answer choices, state the gap in your own words. A good prephrase is specific enough to eliminate wrong answers and flexible enough to match the correct choice. For the bike lane argument, a prephrase might be "people will actually switch from cars to bikes." That captures the central gap without predicting the exact wording.
Prephrasing also protects you from answer choices that are true but irrelevant. The GMAT includes choices that are supported by common sense but do not affect the argument. If a choice does not connect the evidence to the conclusion, it is not the assumption.
Many assumptions repair a mismatch between what the evidence shows and what the conclusion claims. The evidence may describe a correlation, while the conclusion claims causation. The assumption is that no other cause explains the correlation. The evidence may describe a sample, while the conclusion describes a larger population. The assumption is that the sample represents the population.
Example. Argument: Students who eat breakfast score higher on tests. Therefore, eating breakfast improves test scores.
The evidence is a correlation. The conclusion is causal. The argument assumes that breakfast itself causes the higher scores, not that higher-achieving students are more likely to eat breakfast. The negation test confirms this: if high-achieving students are simply more likely to eat breakfast, the causal claim fails.
Pay attention to the scope of the conclusion. If the conclusion says "all," the assumption must cover all. If it says "most," the assumption must cover most. An answer that is too weak may not be enough, and an answer that is too strong may be unnecessary.
Degree words matter. "Some," "many," "most," and "all" are not interchangeable. An assumption that says "some people prefer bike lanes" is weaker than the argument needs if the conclusion claims a large reduction in congestion. A necessary assumption is the minimum the argument requires, not a stronger claim that would make the argument more convincing.
Recommendation arguments assume that the proposed action will produce the desired result and that the result is worth the cost. If a passage recommends a policy because it worked in another city, the argument assumes that the two cities are similar enough for the policy to transfer. A necessary assumption is that the relevant conditions are comparable, not that the cities are identical.
Recommendations also assume that no serious side effect will outweigh the benefit. An answer that rules out a major harm can be a necessary assumption, while an answer that says the policy is popular is usually not. Popularity does not guarantee effectiveness.
For recommendation arguments, ask what must be true for the action to work and for the benefit to justify the cost. The correct assumption often addresses one of those two links.
Argument: A company switched to a four-day workweek and productivity rose. Therefore, other companies should adopt a four-day week to raise productivity.
The conclusion is a recommendation for other companies. The evidence is one company's experience. The gap is whether the result will transfer. A necessary assumption is that the productivity rise was not caused by a factor unique to the first company, such as a temporary surge of enthusiasm or a change in management. If the rise was unique, the recommendation fails.
An answer such as "the four-day week is popular with employees" is not necessary. Employees could dislike the schedule and still be more productive. An answer such as "the first company's productivity gain was not due to a simultaneous change in its production process" is necessary, because that alternative cause would explain the result without the shorter week.
| Question | What to do |
|---|---|
| What is the conclusion? | Find the claim the author wants accepted |
| What is the evidence? | Find the facts offered in support |
| What is the gap? | State the unstated link in your own words |
| Is the choice necessary? | Negate it and see whether the argument fails |
| Is the choice in scope? | Check that it affects the conclusion, not a side issue |
The right assumption is often modest. It does not prove the conclusion, and it does not need to be the most important idea in the argument. It only needs to be a belief the argument cannot survive without.
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