Some doctors believe that a certain drug reduces the duration of episodes of vertigo, claiming that the average durat...

Farnoush on September 26, 2019

Why A and not C?

Hi could you explain why C isn't the answer. I thought both strengthened it and then when I negated I thought C would make the answer choice fall apart. Thank you

3 Replies

on September 26, 2019

Hello @farnoushsalimian,

Keep in mind that a required assumption does not strengthen an argument. Rather, the argument depends entirely on this assumption. This is why the strategy of negating answer choices is effective.

Premise: During a three-month shortage of the drug, there was no significant change in the average duration of vertigo.

Conclusion: The drug has no effect on the duration of vertigo.

The author gives only one piece of evidence to support the conclusion, and that is the three-month window in which the shortage did not have a significant effect. We should know before even looking at the answer choices, that the author is assuming that this piece of evidence is sufficient to support the conclusion.

This is best reflected by answer choice A. It says that if the drug did work, then the three-month shortage would be enough time to see the positive effects of the drug diminish. This didn't happen, so the drug is ineffective. This is necessary for the success of the argument.

You are correct that C probably strengthens the argument. Let's negate it. Even if a shortage greater than three months would have given better information, that does not mean that the data from the three month shortage is worthless. It could still be strong enough to support the conclusion. The argument does not fall apart.

on July 23 at 03:53PM

Why is D not correct? Changes in diet and smoking ARE responsible for any change in the average duration. That would put into question the effectiveness of the measurement of the duration of the episodes?

How would you negate A? A to me seemed like a sufficient assumption answer?

Shunhe on July 28 at 04:36PM

Hi @Anna2020,

Thanks for the question! So first, let’s talk about why (D) is wrong. The negation is actually a bit different from what you did, since negating “not responsible for anything” is just “is responsible for something.” So the correct negation of (D) would be that changes in diet and smoking habits are responsible for SOME of the change in the average duration of vertigo since the introduction of the drug. And using this negation, it’s easier to see why (D) is wrong. The argument still holds even if some of the change was because of changes in diet and smoking. Since we aren’t told exactly how changes in diet and smoking would’ve changed the duration of episodes of vertigo, (D) doesn’t really hurt the argument without further information, and so it’s not a required assumption.

The negation of (A) would be that if a drug made a difference in the duration of vertigo, then it’s possible that a three-month shortage of that drug wouldn’t have caused a significant change in the average duration of vertigo. But if that’s true, then we obviously can’t get to the conclusion, and that’s why (A) is a necessary assumption and the correct answer.

Hope this helps! Feel free to ask any other questions that you might have.