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Difficulty: EasyCausal Flaws and Confounding Variables

A municipality installed bright LED street lighting along main thoroughfares last year. Over the same period, night-time traffic accidents on those thoroughfares decreased by 15 percent. The mayor concluded that the improved visibility from the new street lights directly caused the reduction in night-time traffic accidents. Which of the following, if true, most seriously weakens the mayor's argument?

  1. During the same period, the municipality significantly increased police speed-enforcement patrols on those thoroughfares, resulting in lower driving speeds.Answer
  2. B
    Night-time traffic accidents on nearby unlit rural roads increased over the same one-year period.
  3. C
    The installation cost of the LED street lights exceeded the municipality's original budget allocations.
  4. D
    The new LED street lights consume less electrical energy than the older sodium vapor lights they replaced.
  5. E
    Reducing traffic accidents is a primary goal for municipal transportation departments nationwide.

Answer

The mayor's argument is most seriously weakened by the statement that the municipality significantly increased police speed-enforcement patrols on those thoroughfares during the same period, resulting in lower driving speeds.
The conclusion asserts a direct causal link between enhanced street lighting and a reduction in night-time accidents. The correct response introduces a confounding variable—increased police speed-enforcement patrols during the exact same timeframe. This alternate factor provides a plausible competing explanation for why driving speeds dropped and accidents decreased, directly undermining the mayor's assertion that the lights were the sole cause.

Step-by-Step Solution

1
Identify the conclusion and premises in the argument.
Premise: LED lights were installed, and night-time accidents fell by 15%. Conclusion: Improved visibility from the lights directly caused the decrease in accidents.
Isolating the causal assertion allows us to target the logical gap between the correlation and the conclusion.
2
Identify the causal flaw.
The argument assumes correlation implies direct causation without considering third-variable confounders.
Causal claims on the GMAT are vulnerable to alternate explanations that occurred during the same timeframe.
3
Evaluate the options for a confounding variable.
The choice introducing increased police patrols provides an alternate cause for the reduced accident rate, undermining the claim that lights were responsible.
Demonstrating that another factor could account for the effect weakens the author's primary causal claim.

Key Concept

Causal Flaws and Confounding Variables
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Topics

Causal Flaws and Confounding Variables

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