Bus Frequency, Ridership, and a New Campus
A city transit authority added four extra buses per hour on Route 7, raising service from six to ten buses per hour. Over the next six months, average weekday ridership on Route 7 rose from 4,200 to 4,956, an 18% increase. The authority's planning director concludes that the frequency increase caused the ridership gain and proposes the same increase on Route 12, where buses currently run every 20 minutes. A critic notes that during the same period a large employer opened a new office campus within walking distance of Route 7, and the transit authority also ran a discounted-fare pilot on all routes. The director responds that the fare pilot affected every route, so it cannot explain why Route 7 specifically gained riders. The authority must decide whether to approve the Route 12 increase based on this evidence. It has budget for only one service change this year.
1. Identify the unstated assumption the director must rely on to infer that the Route 7 frequency increase caused the ridership gain.
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The director needs the assumption that no Route 7-specific factor other than frequency materially caused the gain. The fare pilot is a common shock across routes, so it is weak as an explanation for why Route 7 rose relative to others, but it does not neutralize the new employer campus, which is specific to Route 7. The director must also assume that the ridership comparison is not distorted by seasonal effects, route closures, or a change in how ridership is counted.
In short, the argument treats a before-and-after rise on one route as a controlled experiment. It is not controlled: the new campus is a plausible Route 7-specific cause. The conclusion also assumes Route 12 would respond similarly, which requires comparable demand and land-use conditions.
2. Which additional piece of evidence would most weaken the director's conclusion? Explain why.
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Most weakening evidence would show that the new campus, not frequency, supplied most of the added riders. For example, a survey finding that 700 of the 756 net new Route 7 riders work at the new campus would undercut the causal claim. That would leave only 56 riders potentially attributable to frequency, far below the 18% gain.
A second strong weakening item would be ridership records showing Route 7 was already rising sharply before the frequency change, or that a parallel route without added buses gained similarly. Either pattern suggests the gain was not caused by the four extra buses. The key is to tie the evidence to a Route 7-specific alternative cause or to the timing of the change.
3. The director says the fare pilot cannot explain Route 7's gain because it affected all routes. Is this response sufficient? Justify your answer.
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The response is partially valid but not sufficient. Because the fare pilot applied to all routes, it cannot by itself explain why Route 7 gained while other routes did not. That is a useful control.
However, the response does not address the new campus, which is Route 7-specific. It also assumes the pilot had the same effect on every route; if Route 7 riders were more fare-sensitive, or if the discount and higher frequency reinforced each other, the pilot could still contribute to the gain. The director needs evidence separating the campus effect from the frequency effect before the Route 12 proposal is well supported.