In the nation of Partoria, large trucks currently account

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In the nation of Partoria, large trucks currently account for 6 percent of miles driven on Partoria's roads but are involved in 12 percent of all highway fatalities. The very largest trucks-those with three trailers-had less than a third of the accident rate of single- and double-trailer trucks. Clearly, therefore, one way for Partoria to reduce highway deaths would be to require shippers to increase their use of triple-trailer trucks.

Which of the following, if true, most seriously weakens the argument?

(A) No matter what changes Partoria makes in the regulation of trucking, it will have to keep smaller roads off-limits to all large trucks.

(B) So far only the best, most experienced drivers for Partorian trucking companies have been driving triple-trailer trucks.

(C) Very few fatal collisions involving trucks in Partoria are collisions between two trucks.

(D) In Partoria, the safety record of the trucking industry as a whole has improved slightly over the past ten years.

(E) In Partoria, the maximum legal payload of a triple-trailer truck is less than three times the maximum legal payload of the largest of the single-trailer trucks.

OA B

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by ceilidh.erickson » Thu May 16, 2019 11:12 am

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There are several logical flaws in this argument:

Premises: Trucks with three trailers had less than a third of the accident rate of single- and double-trailer trucks

Conclusion: one way for Partoria to reduce highway deaths would be to require shippers to increase their use of triple-trailer trucks.

Unstated assumptions:
- assuming accident rate is directly tied to fatality rate. But what if 3-trailer trucks are involved in fewer accidents, but their larger size makes those accidents more likely to result in fatalities?
- assuming all circumstances are equal for 3-trailer v. 1- or 2-trailer trucks. But what if 3-trailer trucks tend to be driven on better roads, or are generally driven during the day or in better weather conditions? What if they're driven by different people?

To weaken the argument, we want to find an answer choice that undermines one of these unstated assumptions.


(A) No matter what changes Partoria makes in the regulation of trucking, it will have to keep smaller roads off-limits to all large trucks.
- this would impact any of the trucks, so it doesn't hurt the conclusion that more 3-trailer trucks would reduce highway deaths.

(B) So far only the best, most experienced drivers for Partorian trucking companies have been driving triple-trailer trucks.
- this undermines the idea that all circumstances are equal for 3-trailer v. 1- or 2-trailer trucks. If the best drivers drive 3-trailer trucks, then the lower fatalities could be explained by this human factor, not by the trucks themselves. If we put more 3-trailer trucks on the road, some of them would have to be driven by less-experienced drivers - presumably the same drivers who were likely causing the fatalities with the 1- and 2-trailer trucks.

(C) Very few fatal collisions involving trucks in Partoria are collisions between two trucks.
- the argument doesn't specify whether fatalities were truck-on-truck or car-on-truck, so this doesn't matter.

(D) In Partoria, the safety record of the trucking industry as a whole has improved slightly over the past ten years.
- this could be true, and it could still be true that switching to a safer truck would further reduce fatalities. This doesn't weaken.

(E) In Partoria, the maximum legal payload of a triple-trailer truck is less than three times the maximum legal payload of the largest of the single-trailer trucks.
- we don't care about the specifics of these trucks. This does not undermine the idea that more 3-trailers will be safer.

The answer is [spoiler]B.
[/spoiler]
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education