Necessary and Sufficient Assumptions Flashcards

Argument: 'The unemployment rate fell last quarter. When unemployment falls, consumer confidence rises. Therefore, consumer confidence rose last quarter.' A critic challenges: 'Other factors may have suppressed consumer confidence.' The most effective response relies on which assumption?

Answer: No other factor was strong enough to override the effect of falling unemployment on confidence

The argument needs to assume that the relationship between unemployment and confidence wasn't overridden by other forces last quarter.

On the LSAT, which question stem signals a Necessary Assumption question?

Answer: 'The argument assumes which of the following?'

'The argument assumes' directly asks for a necessary assumption—something the argument is already implicitly relying on.

Which of the following is a sufficient assumption for: 'Maria scored higher than everyone in her class on the entrance exam, so Maria will be admitted to the program'?

Answer: The program admits the student with the highest entrance exam score in any applicant pool

If the program always admits the top scorer, then Maria's highest score guarantees her admission, making this sufficient.

An argument concludes that a policy change caused a drop in crime. A necessary assumption is that:

Answer: No other simultaneous change could account for the drop in crime

A causal argument requires ruling out alternative causes; otherwise the policy change might not be responsible for the drop.

A necessary assumption and a sufficient assumption can both be true for the same argument. How do they differ in function?

Answer: The necessary assumption is required for the argument to work; the sufficient assumption is strong enough to guarantee the conclusion

Necessary assumptions are the minimum required; sufficient assumptions provide enough to logically secure the conclusion—both can coexist.

Argument: 'This restaurant received a health code violation last year. I will not eat there.' Which necessary assumption does this argument rely on?

Answer: A past health code violation indicates current or ongoing risk to diners

The argument moves from a past violation to a current decision, so it must assume the past violation is still relevant to present risk.

A law school applicant argues: 'I have a high GPA and LSAT score. Top law schools admit applicants with high GPAs and LSAT scores. Therefore, I will be admitted to a top law school.' A necessary assumption is:

Answer: The applicant's GPA and LSAT are high enough to meet the threshold those schools use

The argument requires that the applicant's scores meet whatever standard the schools consider 'high'—otherwise the general premise doesn't apply.