21. CR Assumption
Negation test: negate the choice; if the argument collapses, it's the assumption.
Core ideas
- An assumption is an unstated premise the argument needs to be true; it bridges evidence to conclusion.
- Negation test: negate the answer choice; if the negated version destroys the argument, that choice is the assumption.
- Negate logically ("at least one does not"), not to the extreme opposite.
- Necessary means required, not helpful; a choice can strengthen without being assumed.
Worked example 1
Brindle Foods will replace the plastic trays in its frozen meals with compostable fiber trays. Since fiber trays cost less to produce than plastic ones, Brindle's profit per meal will increase.
Which is an assumption the argument requires?
(A) Compostable trays are better for the environment than plastic trays.
(B) Switching to fiber trays will not increase Brindle's other per-meal costs, such as machinery changes or spoilage, by more than the savings on trays.
(C) Consumers prefer compostable packaging.
Show solution
The conclusion is about profit per meal; the evidence is only about tray production cost. The gap: cheaper trays raise profit only if nothing else eats the savings. Negate (B): the switch DOES raise other per-meal costs by more than the tray savings. Then profit per meal falls and the conclusion collapses, so (B) is required. Negating (A) or (C) leaves the cost-to-profit logic intact; environmental benefit and consumer preference are never part of this argument. Answer: (B).
Worked example 2
The Larkin Museum plans to extend Friday hours to midnight to increase total weekly attendance, reasoning that people who work daytime shifts will visit in the late evening.
Which is an assumption the plan requires?
(A) At least some late-evening visitors will be people who would not otherwise have visited during regular hours that week.
(B) Most museum visitors work daytime shifts.
(C) Other museums that extended hours saw attendance rise.
Show solution
The goal is higher TOTAL weekly attendance, not just full evening galleries. Negate (A): every late-evening visitor would have come during regular hours anyway. Then evening visits merely shift existing attendance around and the weekly total does not rise, so the plan fails; (A) is necessary. (B) is too strong: the plan needs only some new visitors, not that most visitors work days. (C) would mildly strengthen but is not required for this museum's logic. Answer: (A).
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