20. CR Strengthen / Weaken
Find the conclusion, attack or support the gap between evidence and conclusion.
Core ideas
- First, find the exact conclusion and the evidence; the right answer works on the gap between them, not on either alone.
- Strengthen: rule out an alternative explanation, confirm the causal link, or show the evidence is representative.
- Weaken: supply an alternative cause, show the sample or comparison is broken, or show the plan backfires.
- Do not attack the evidence itself; GMAT treats stated premises as true.
Worked example 1
The city of Varenport installed brighter streetlights on Harbor Avenue last year, and nighttime traffic accidents on that street fell by 30 percent. The city council concludes that installing the same lights citywide will reduce nighttime accidents across Varenport.
Which option most weakens the council's conclusion?
(A) Daytime accidents on Harbor Avenue were unchanged.
(B) During the same year, a nearby highway opened, diverting most nighttime through-traffic away from Harbor Avenue.
(C) The new lights cost twice as much as the old ones.
Show solution
The conclusion is causal: the lights caused the drop, so lights elsewhere will cause drops elsewhere. The gap is whether the lights, rather than something else, produced the 30 percent decline. Choice (B) supplies an alternative cause: fewer cars drove Harbor Avenue at night, which alone could explain fewer accidents. If traffic volume fell, the lights may have done nothing, and citywide installation would not help. (A) is consistent with the lights working (they only matter at night), and (C) is about cost, not effectiveness. Answer: (B).
Worked example 2
A study at Delmore University found that students who ate breakfast scored higher on morning exams than students who skipped it. The dean concludes that providing free breakfast will raise morning exam scores.
Which option most strengthens the dean's conclusion?
(A) Students who eat breakfast also tend to sleep more hours.
(B) In a pilot program, habitual breakfast-skippers who were randomly assigned free breakfast improved their morning exam scores, while skippers not assigned breakfast did not improve.
(C) Breakfast is the most popular meal among Delmore students.
Show solution
The evidence is a correlation; the conclusion is causal (breakfast raises scores). The gap: maybe breakfast-eaters differ in other ways, such as discipline or sleep. Choice (B) closes that gap with a randomized comparison: when breakfast itself is added to otherwise similar skippers, scores rise, which isolates breakfast as the cause. (A) actually weakens by suggesting a confounding variable, and (C) is irrelevant popularity. Answer: (B).
No practice questions banked for this concept yet. Check back soon.