Assumptions and Gaps
Key Takeaways
- A gap is the distance between what the premises establish and what the conclusion claims.
- Necessary assumptions must be true for the argument to survive; sufficient assumptions prove the conclusion when added to the premises.
- Bridge assumptions connect new terms, while defender assumptions block threats such as alternate causes or exceptions.
- Useful prephrases name the missing link in functional terms instead of trying to predict exact answer wording.
What The Gap Is
An LR argument rarely proves its conclusion completely. The gap is the missing support between the stated evidence and the claim the author wants accepted. The test often asks you to find, protect, attack, or describe that missing support.
This is not a trick. LSAC's Logical Reasoning section is built around analyzing and evaluating short arguments. In practice, that means noticing when a premise is true but still not enough.
A gap is easiest to see by comparing terms. Premise: residents downloaded a city app. Conclusion: residents are now better informed about city services. The new idea is being informed. The argument assumes app downloads led to actual use or knowledge.
Common Gap Sources
| Premise | Conclusion | Gap to inspect |
|---|---|---|
| Study participants improved | Program caused improvement | control group, self-selection, alternate cause |
| Expert recommends policy | Policy is best | expertise, evidence quality, possible bias |
| Similar plan worked elsewhere | Plan will work here | relevant similarities and differences |
| Survey respondents prefer X | Public prefers X | representative sample |
| Rule applies to one category | Specific case is covered | category membership |
| Percent increased | Amount increased meaningfully | base numbers |
The best prephrase is short and functional. Say alternate cause, category bridge, sample problem, new term, base rate, or relevant difference. You do not need the final wording yet.
Necessary Assumptions
A necessary assumption is something the argument requires. It may be modest. It may not prove the conclusion. Its job is survival.
The standard test is negation. Negate the answer and ask whether the argument is seriously damaged. If the argument can still work, the original answer was not required. If the argument collapses, the original answer is necessary.
Negate carefully. Some becomes none. All becomes not all. More likely becomes not more likely. Do not negate into an exaggerated opposite unless the wording requires it.
Necessary assumptions often sound boring because arguments usually need basic conditions. If a policy will reduce traffic because commuters will use a new bus line, the argument may need the assumption that at least some commuters can access the line.
Sufficient Assumptions
A sufficient assumption is enough to guarantee the conclusion when combined with the premises. These answers are often stronger than necessary assumptions. They close the gap completely.
Look for new terms. If the premise says all certified inspectors completed training and the conclusion says Jordan is qualified to review the site, the sufficient bridge might say anyone who completed that training is qualified and Jordan is a certified inspector.
Sufficient-assumption stems reward exact matching. A tempting answer that helps the conclusion may still fall short. Ask whether the premises plus the answer force the conclusion, not merely make it plausible.
Necessary Versus Sufficient
| Task | Answer must | Typical force | Test |
|---|---|---|---|
| Necessary assumption | Be required by the argument | Often modest | Negate and see whether support fails |
| Sufficient assumption | Prove the conclusion | Often strong | Add it to premises and check whether conclusion follows |
| Strengthen | Make support better | Partial is enough | Confidence increases |
| Weaken | Make support worse | Partial is enough | Confidence decreases |
Bridge And Defender Assumptions
A bridge assumption connects language from premises to language in the conclusion. These are common when the conclusion introduces a new standard, group, or result.
A defender assumption rules out a threat. It might say the sample was not biased, the measurement did not change, the effect was not caused by a different factor, or no exception applies.
Both types are exam-useful. Bridge assumptions are visible through new terms. Defender assumptions are visible through vulnerabilities. If the argument says a drug helped because patients improved after taking it, a defender might rule out a simultaneous treatment.
Using Gaps Across Question Types
For strengthen, find something that supports the missing link. For weaken, find something that damages it. For flaw, describe the failure. For evaluate, ask which information would test it in either direction.
For example, an argument claims a tutoring platform caused higher scores because users improved after subscribing. A strengthener could show nonusers with similar starting scores did not improve. A weakener could show subscribers also received extra classroom instruction. A flaw answer could say the argument treats a before-after pattern as proof of cause without ruling out other explanations.
The subject changes; the gap family remains. That is why assumption work is a foundation rather than a separate topic.
Review Method
After each missed argument question, write one sentence: because P, the author concludes C, assuming G. If G is vague, keep working. A useful gap can be tested.
Then classify the answer. Did it bridge terms? Defend against a threat? Introduce an alternate cause? Show a relevant difference? Supply a rule? Correct review turns every missed question into a reusable diagnostic.
Prephrase Without Overcommitting
A prephrase should guide evaluation without blinding you. If you predict alternate cause, still consider an answer that attacks measurement or sample selection if it damages the same causal support. The label is a starting point, not a requirement that the credited answer use your exact route.
This matters on harder LR items because one gap can be described several ways. Stay loyal to the conclusion and support, not to the first phrase that came to mind.
The LSAT does not require you to distrust every fact. It requires you to ask whether the accepted facts actually support the conclusion. That difference is the heart of assumption work.
Analyst: A new online filing system was introduced in April. In May, clerks processed 30 percent more permit applications than in March. Therefore, the online filing system made the clerks more productive. Which assumption is most necessary to the argument?