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Difficulty: Very hardRuling Out Alternate Explanations (Defender)

Over the past decade, the municipality of Oakhaven implemented a 'Green Streets' initiative, significantly increasing the number of trees and public parks in its downtown district. During this same ten-year period, the incidence of stress-related illnesses reported by downtown residents decreased by 30 percent. Because clinical studies have consistently demonstrated that exposure to urban green spaces lowers cortisol levels, city officials conclude that Oakhaven's newly added green spaces directly caused the local reduction in stress-related illnesses.

Which one of the following is an assumption on which the city officials' argument depends?

  1. A
    If the 'Green Streets' initiative had not been implemented, there would have been no decrease in the incidence of stress-related illnesses in Oakhaven's downtown district.
  2. B
    Every downtown resident who reported a decrease in stress-related illness during the past decade regularly spent time in the newly added public parks and green spaces.
  3. The 'Green Streets' initiative did not lead to increases in the cost of living that disproportionately drove residents highly susceptible to stress-related illnesses out of the downtown district.Answer
  4. D
    The incidence of stress-related illnesses among Oakhaven's downtown residents was already steadily declining prior to the commencement of the 'Green Streets' initiative.
  5. E
    Other municipalities that have implemented similar initiatives have experienced reductions in stress-related illnesses comparable to those seen in Oakhaven.

Answer

The argument fundamentally relies on the assumption that the 'Green Streets' initiative did not lead to increases in the cost of living that disproportionately drove residents highly susceptible to stress-related illnesses out of the downtown district.
The author concludes that the new green spaces directly caused the 30 percent reduction in stress-related illnesses based on a correlation in time. This causal claim requires assuming that there isn't an alternative explanation for the drop in the illness rate among downtown residents. The correct answer acts as a Defender assumption by ruling out a demographic shift (selection bias). If we apply the Negation Technique and assume the opposite—that the initiative DID cause living costs to rise, thereby driving highly susceptible residents out—then the district's illness rate would drop simply because the sickest people relocated. This alternative explanation destroys the conclusion that the green spaces themselves were the medical cause of the statistical change. Therefore, the argument absolutely depends on assuming this demographic shift did not happen.

Step-by-Step Solution

1
Analyze the stimulus to separate the premises from the main conclusion.
Premises: Green spaces increased, illness incidence decreased by 30%, clinical studies show green spaces lower cortisol. Conclusion: Green spaces directly caused the reduction.
Identifying the exact causal claim is necessary to pinpoint logical vulnerabilities.
2
Identify the core logical flaw in the argument's reasoning.
The argument assumes causation based on a temporal correlation and ignores population dynamics, assuming the 'downtown residents' demographic remained stable.
An epidemiological drop in incidence can occur either through actual healing or through demographic shifts (selection bias).
3
Prephrase the type of assumption required to defend the argument.
The argument requires a 'Defender' assumption that rules out potential alternative causes for the statistical drop, such as a changing population.
Because the conclusion makes a definitive causal claim, it inherently assumes that other confounding variables did not produce the effect.
4
Apply the Negation Technique to evaluate the options.
Negating the correct option yields: 'The initiative DID lead to increases in living costs that disproportionately drove highly susceptible residents out.' This alternate cause perfectly explains the 30% drop, destroying the argument.
The Negation Technique provides a mathematical proof for necessary assumption questions. If the negated statement ruins the conclusion, the original statement is strictly required.

Key Concept

Ruling Out Alternate Explanations (Defender)
Estimated Time:2m 30s
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Ruling Out Alternate Explanations (Defender)

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