GMAT Focus EditionVerbal ReasoningEasy
A new study reveals that cities with a higher number of public parks per capita experience lower rates of respiratory illnesses among their residents. Researchers conclude that increasing the number of public parks directly improves public health. Which of the following, if true, would most strengthen the researchers' conclusion?
- AThe types of trees and plants in public parks are known to absorb common air pollutants.
- BResidents living near public parks are more likely to engage in outdoor physical activity.
- CThe study controlled for population density and average income levels of the cities analyzed.
- DCities with more public parks also tend to have more stringent air quality regulations.
Show answer & explanationAnswer & explanation
Correct answer: C. The study controlled for population density and average income levels of the cities analyzed.
To strengthen a causal claim, one must rule out alternative explanations or confounding factors. By controlling for population density and income, the study becomes more robust in isolating the effect of parks, making it more likely that the parks themselves are the cause, rather than these other factors.
Why the other options are wrong
- A. This provides a mechanism for how parks *could* improve health, but it doesn't strengthen the *causal link* from the study's correlation; it's a plausible explanation, but not a strengthening of the study's methodology.
- B. While physical activity is beneficial, this option describes an indirect benefit of parks, not a direct improvement in respiratory health from the parks themselves.
- D. This suggests an alternative cause (air quality regulations) for lower respiratory illnesses, thus weakening, not strengthening, the conclusion that parks directly improve health.
Strengthening Causation
To strengthen a claim that A causes B, provide evidence that removes alternative explanations, confirms the mechanism, or shows a direct link.
- Rule out confounding variables.
- Establish a plausible mechanism.
- Demonstrate consistency across different contexts.
Memory trick: Build up the bridge, remove the fog, confirm the plot, and the argument is solid rock!