First, let me address your concerns regarding the question stem. Flaw and Weaken questions have a lot in common: if you can understand how to weaken the argument, then fundamentally you have some grasp of the flaw in it, and you can use that information to help you determine the correct answer choice. In other words, if you know a weakness in an argument, you are capable of seeing a concrete way to attack the argument, and this reveals, to some extent, the abstract nature of the flaw that is present.
To answer your specific question from PT55, Section I, Question 24, I'm inclined to treat this as a Weaken question, even though the stem is, technically, a Flaw stem ("the reasoning is most vulnerable to criticism on the grounds that the advertisement overlooks the possibility that..."). Why can I do this? Arguments frequently fail to take into account any number of different possibilities, most of which are entirely irrelevant to the logical validity of the conclusion. A discrete number of possibilities, however, are relevant and should have been taken into account because, if true, they could potentially weaken the argument.
Only possibilities that could potentially weaken the argument are possibilities that the author should have considered, and whose omission amounts to a logical flaw!
The advertisement recommends a diet which provides more calories from protein than from anything else, and which requires that breakfast be the biggest meal of the day. The author argues that anyone who follows this diet is sure to lose weight, because of a study showing that those who lost the
most weight had a similar diet to the one being advertised. Let's call it the Atkins diet
This is a terrible line of reasoning. Just because the richest man in the world is in computer technology doesn't mean that anyone who get into computer technology is sure to become rich. Likewise, just because those who lost the
most weight followed the Atkins diet doesn't mean that the diet will work for everyone. Maybe the "biggest losers" followed that diet, but how many of those who followed the Atkins diet actually lost weight? That's what I'd like to know... otherwise, imagine the following example:
You have 6 people (A, B, C, D, E, F) in a study comparing the effectiveness of the Atkins diet to that of the Mediterranean diet. 3 people were randomly assigned to each group. The results are as follows, with a designation of the weight each person lost:
Atkins:
A - 10 lbs
B - 0 lbs
C - 0 lbs
Mediterranean
D - 9 lbs
E - 9 lbs
F - 9 lbs
As you can see, the biggest loser followed the Atkins diet, suggesting that the Atkins diet can - in theory - cause one to lose more weight than the other diet. But which of the two diets is more
likely to cause a random person to lose weight? Obviously, the Mediterranean - 100% of those who followed it lost some weight. By contrast, two of the three people on the Atkins lost no weight at all, suggesting that a random person following the Atkins diet is more likely not to lose weight (66%) than to lose weight (33%).
The advertisement clearly overlooks the importance of understanding what
proportion of those who followed their diet lost weight, so that you can make an accurate prediction of your likelihood of losing weight by following their diet. The fact that the biggest loser followed the Atkins diet only means that the diet can
potentially cause one to lose more weight than any other diet can, not that
anyone is sure to lose weight following that diet.
Does that help you understand better why answer choice (D) describes a possibility that would weaken the argument?
Answer choice (C) would only be attractive if you misunderstood the language in the conclusion. The advertisement never promised that following the Atkins diet guarantees losing the
most weight possible. Even if there were a way to lose more weight, the conclusion would still stand: it is still entirely possible that anyone who follows Atkins would lose weight, just not as much as they would lose if they also exercised.
Does that make sense? Let me know.
Thanks!
