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#2 - We already knew from thorough investigation that

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Complete Question Explanation

Evaluate the Argument. The correct answer choice is (A)

This stimulus presents information about a traffic accident. From prior investigation, it is known that at least one of the following factors was in play: The driver of the first vehicle did not signal prior to changing lanes, and/or the driver of the second vehicle was speeding. Either of these two would make a driver liable for the resulting accident, as follows:

    Driver One signaled :arrow: Driver Two speeding :arrow: Driver Two Liable

    Driver Two speeding :arrow: Driver One signaled :arrow: Driver One Liable

It is known that the driver of the first vehicle was changing lanes without signaling. Based on this premise, the author of the stimulus improperly concludes that the driver of the second vehicle must not be liable, based on the following flawed reasoning:

    Driver One signal :arrow: Driver One Liable :arrow: Driver Two Liable

This is a mistaken negation of the first conditional rule diagrammed above. We do know that if Driver Two wasn’t speeding, then Driver One must have been at fault, but we cannot logically conclude that if Driver One was at fault, that Driver Two must not have been speeding.

The conclusion that Driver Two is liable is flawed, because the conditional reasoning in the stimulus would allow for shared liability—that is, we know that if one driver isn’t liable, the other one must be, but we cannot conclude the reverse: that if one driver is liable, the other one isn’t. The question stem asks which answer choice provides the most important information for evaluating the author’s conclusion. As we can see from the original conditional statements offered in the stimulus, the only provided method to determine Driver Two’s liability concerns that driver’s speed.


Answer choice (A): This is the correct answer choice. If the driver of the second vehicle was driving at an excessive speed, then we could properly conclude that the second driver shares in the liability:

    Driver Two speeding :arrow: Driver Two Liable

This is the only information, based on the conditional reasoning in the stimulus, that would allow us to draw such a conclusion.

Answer choice (B): The first driver’s knowledge of the infraction is immaterial to the question of causality. Unwitting or not, it is the failure to signal that would attach liability for the resulting accident.

Answer choice (C): The involvement of other drivers is also immaterial to the question of liability on the part of the two drivers that we know were involved. The stimulus tells us that “it is known” that either the driver of the first vehicle did not properly signal a lane change or that the driver of the second vehicle was speeding, and that either of these would result in a driver’s liability for the resulting accident. The involvement of other vehicles is not relevant to the conditional reasoning provided in the stimulus.

Answer choice (D): If a witness is unreliable, then any conclusions based on premises this witness establishes might also be unreliable. But the stimulus states that “further evidence has proved that the turn signal was not on, though the driver of that vehicle admits to having changed lanes.” The reliability of the first driver is therefore not at issue, and knowledge of the information from this answer choice would lend no insight into the evaluation of the author’s conclusion.

Answer choice (E): The stimulus specifically states that, absent excessive speed by the second driver, failure to signal would be the sole cause of the resulting accident. Thus, whether the second driver would have seen the signal is irrelevant.