Critical Reasoning

Manufacturers of motor oil for car engines have found through research that motor oil slowly loses lubrication effectiveness after the car is driven for certain distances. On average, the motor oil is only fifty percent as effective after 3,000 miles of driving as it is the first day it is put into the car. For that reason, people should replace their motor oil every 3,000 miles.

Which of the following, if true, would most weaken the conclusion above?

A. The research indicates that the oil loses effectiveness because it becomes dirty.
B. The researchers found that there was not a significant difference in the effectiveness of motor oil in cars that reached 3,000 miles quickly versus those that took a long time to reach 3,000 miles.
C. The researchers found that cars with motor oil at fifty percent effectiveness showed no more signs of damage or wear than those with fresh motor oil.
D. The researchers were paid by the manufacturers of motor oil, who have a vested interest in selling more motor oil.
E. Both synthetic and non-synthetic motor oils experience similar losses in motor oil effectiveness after 3,000 miles of driving.

thetrystero wrote:

Manufacturers of motor oil for car engines have found through research that motor oil slowly loses lubrication effectiveness after the car is driven for certain distances. On average, the motor oil is only fifty percent as effective after 3,000 miles of driving as it is the first day it is put into the car. For that reason, people should replace their motor oil every 3,000 miles.

Which of the following, if true, would most weaken the conclusion above?

A. The research indicates that the oil loses effectiveness because it becomes dirty.
B. The researchers found that there was not a significant difference in the effectiveness of motor oil in cars that reached 3,000 miles quickly versus those that took a long time to reach 3,000 miles.
C. The researchers found that cars with motor oil at fifty percent effectiveness showed no more signs of damage or wear than those with fresh motor oil.
D. The researchers were paid by the manufacturers of motor oil, who have a vested interest in selling more motor oil.
E. Both synthetic and non-synthetic motor oils experience similar losses in motor oil effectiveness after 3,000 miles of driving.

My answer: D

Situation:
1. motor oil is 50% as effective after 3000 miles.
Therefore, replace it every 3000miles.

A: cause of its losing its effectiveness is irrelevant
B: in a way, this strengthens the argument because it shots down one factor for doubt.
C: If the researchers are acting in the interest of the oil companies, all of their claims would be equally suspect.
D: Validity of premise 1 is would be suspect.
E: This just says that you should change it whether you use synthetic or non-synthetic.

Uri wrote:OE: In this argument, the conclusion is that people should change their motor oil every 3,000 miles. The conclusion is based on the fact that motor oil loses half of its effectiveness after being driven that distance. The central assumption here is that the loss of effectiveness is indeed a valid reason for changing the motor oil. Choice (C) challenges this assumption by saying that cars with motor oil at fifty percent effectiveness do not have any ill effects. Thus, choice (C) is the correct answer.

Choices (A), (B), and (E) are all out of scope. The fact that oil loses effectiveness because it becomes dirty does not weaken the conclusion. It also does not matter that cars that reach 3,000 miles at different rates experience similar declines in motor oil effectiveness; it also does not matter that the same is true of cars with different types of motor oil.
Choice (D) may be tempting, but is ultimately irrelevant. We may want to gravitate to a choice like (D) that questions the motives for the research, however we never want to challenge the evidence itself in a weaken question – only the central assumption. Even though the research was conducted under the supervision of motor oil companies, this does not alone weaken the conclusion that motor oil should be changed every 3,000 miles.

Learnt something new and thought of sharing