#25 - To test the claim that vitamin C is effective in treating acne, scientists

[Administrator]

Complete Question Explanation

Weaken. The correct answer choice is (A).

Answer choice (A): This is the correct answer choice.

Answer choice (B):

Answer choice (C):

Answer choice (D):

Answer choice (E):

This explanation is still in progress. Please post any questions below!

[sarah_tucker@alumni.brown.edu]

Hi, can someone walk through the reasoning behind A? I selected D as the right answer thinking that the vitamin C in food could account for similar results between people who knew which pill they were taking. I also understand how answer A could be correct, but it feels like it rests on similar assumption as D. Can you walk through why A is better than D?

[Luke Haqq]

Hi Sarah!

The key difference between answer choices (A) and (D) is that the former addresses the group that received vitamin C, while the latter addresses the group that received the placebo.

This seems to be an important difference, for the conclusion of the stimulus is, "Therefore, we can tentatively conclude that vitamin C has no real benefit in reducing the severity of acne." Given the question stem, we want to weaken this conclusion, that is, we want an answer choice indicating that vitamin C does have a benefit in reducing the severity of acne.

This is what answer choice (A) does. That answer choice states that "The subjects who were given vitamin C had a history of suffering from more severe acne than did the subjects receiving a placebo." If there was no difference in the severity of the acne between the two groups after the study, yet the group receiving vitamin C had much worse acne from the start, then this suggests that the vitamin C was beneficial in making their acne less severe.

[lemonade42]

Hi, I chose D because if the placebo people also ate vitamin C, then that would mean that you can't compare the two groups anymore, weakening any conclusion derived from them (like the comparison that they show no difference). Because now the two groups (placebo vs. vitamin C treatment) are essentially the same, so now you can't compare the similar groups.

[Dana D]

Hey Lemonade,

The argument (conclusion) here is that vitamin C has no real benefit on reducing the severity of acne. The premise to support that is that there was no difference in the severity of acne amongst the control and test group for those who didn't know what type of pill they were getting.

Remember the question is asking which answer choice most weakens this argument. Answer choice (D) calls the conclusion into question, but some can just mean 1 person, and we don't know if the subjects referenced in this answer choice are people who knew they had placebos or people who didn't know, so it's not as strong as an answer as (A).

Answer choice (A), in comparison, tells us the subjects (meaning all the subjects) who got vitamin C had a history of more severe acne than those getting a placebo. This means that among the people who didn't know what pill they got, the fact that there is no difference in the severity of acne is actually a strong indicator that vitamin C works, because those who got the vitamin C had skill just as good as those who got the placebo, despite their history of more severe acne.

Does this make sense?

[jackieb]

Normally, I would agree that A is a perfect weakener. However, we are told that “the group receiving the vitamin C had less severe acne during the study than the control group.” So if the two groups ended up with the same level of acne, then the placebo improved the most. I thought AC A is trying to get at the same (but reversed) assumption: vitamin C group improved the most because THEY had a more severe history. To me, this is invalidated by the stimulus because I see the level they start out with as being what is important.

[Dana D]

Hey jackieb,

I think you just misread the stimulus here - when it says the Vitamin C group had less severe acne it doesn't mean they started with less severe acne, but rather the use of Vitamin C resulted in them having less severe acne then the control group during the experiment. Historically, (according to answer choice A) this vitamin C group had very severe acne, but improved during the study thanks to the vitamin.

The wording of this is a little confusing, so I understand how that might have gotten lost.

Hope that helps!