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Difficulty: MediumFlaw in the Reasoning

Urban Planner: Whenever a metropolitan transit district achieves an on-time rail reliability rate exceeding 90 percent, private vehicle congestion on surrounding arterial highways declines significantly during peak commuting hours. Over the past twelve months, private vehicle congestion on the arterial highways surrounding Central County declined significantly during peak commuting hours. Thus, Central County's transit district must have achieved an on-time rail reliability rate exceeding 90 percent over the past twelve months.

The reasoning in the urban planner's argument is most vulnerable to criticism on the grounds that the argument

  1. A
    generalizes about commuter patterns across an entire region based on a sample of arterial highways that may be unrepresentative
  2. B
    confuses a claim that serves as an intermediate conclusion with the main conclusion the argument seeks to establish
  3. infers that a condition is required to achieve a particular outcome merely from the claim that the condition is sufficient to produce that outcomeAnswer
  4. D
    bases its conclusion on the claim that traffic survey methodologies failed to accurately measure arterial highway congestion
  5. E
    relies on overly extreme language by claiming that reduced congestion never occurs without increased transit funding

Answer

The argument's flaw is that it infers that a condition is required to achieve a particular outcome merely from the claim that the condition is sufficient to produce that outcome.
The argument assumes that achieving over 90 percent rail reliability is the only possible cause of significant congestion reduction. While the premise guarantees that high rail reliability will reduce congestion (sufficient condition), it does not establish that high rail reliability is necessary to reduce congestion. Thus, concluding that high rail reliability must have occurred because congestion declined is a mistaken reversal.

Step-by-Step Solution

1
Deconstruct the argument into its constituent premises and conclusion.
Premise 1 establishes a conditional rule: If rail reliability exceeds 90 percent, then arterial highway congestion declines significantly. Premise 2 states the factual condition: Arterial highway congestion declined significantly in Central County. Conclusion claims: Central County's rail reliability must have exceeded 90 percent.
Accurately identifying the formal logical relationships in the stimulus is necessary to isolate the structural flaw.
2
Analyze the validity of the conditional inference.
The argument observes that the necessary consequence of the conditional relationship (reduced congestion) has occurred and mistakenly concludes that the sufficient condition (rail reliability exceeding 90 percent) must also have occurred.
This is a classic conditional fallacy known as affirming the consequent or mistaken reversal.
3
Match the identified flaw with the correct abstract description among the choices.
The correct description states that the author treats a condition that is merely sufficient for producing an outcome as if it were necessary (required) for that outcome.
Other factors (such as remote work adoption, fuel price increases, or road expansions) could have caused congestion to drop even if rail reliability remained below 90 percent.

Key Concept

Conditional Reversal (Affirming the Consequent)
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Topics

Flaw in the Reasoning

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