Critical Reasoning

The number of North American children who are obese-that is, who have more body fat than do 85 percent of North American children their age-is steadily increasing, according to four major studies conducted over the past 15 years.

If the finding reported above is correct, it can be properly concluded that

(A) when four major studies all produce similar results, those studies must be accurate
(B) North American children have been progressively less physically active over the past 15 years
(C) the number of North American children who are not obese increased over the past 15 years
(D) over the past 15 years, the number of North American children who are underweight has declined
(E) the incidence of obesity in North American children tends to increase as the children grow older

[spoiler]The OA given is Op C, i have some doubt regarding the reasoning given for Op C. The reasoning given is: the no of children who are obese is increasing as given in Op C plus writer assumes that total no of children also increasing and hence obviously the no of children who not obese is also increasing. Now, i have some doubt in the RED color part...is it really really necessary that "total no of children also increasing", i might be thinking illogically but please clear, isn't it possible that total pop. remains same, may be because of, for example, say a huge no of children who are not obese expired because of some disease etc. if at all that case will happen then i think Op C will not hold as a must be true statement.[/spoiler]

note that the question says the number of children who are obese is increasing. it then defines "obese children" as "children with more body fat than 85 percent of children their age." so obese children will always be 15 percent of the population.

that means that if the number of obese children increased, the total population of children in the same age range must have also increased.

if the total population remained the same, then the number of obese children would not increase, since obesity in this case is defined as a percentile.

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as a separate note, i think it is unlikely that the GMAT will ask you a question in which you have to assume (with no prompt from the question itself) that an unlikely catastrophe occurred.

@atulmangal, @LIL to my thinking, the issue covers not percentage and number

let me switch off the text to highlight precise meaning here

The number of North American children who are obese ... is steadily increasing ...

... children who are obese ... have more body fat than do 85 percent of North American children their age ...

We encounter the number of obese children and the percentage of their fat. Should we compare the number of children with their fat percentage values? no

the obese are not 15% of all children. The obese have simply greater percentage of fat than do 85% of all children their age, BUT it's not given how many children are the obese.

a) defines new scope (parameters, information not found in the text) for studies, BUT we don't know when the studies are ought to be accurate
b) new information has been introduced, not encountered in the text
c) the number of North American children who are not obese increased over the past 15 years(left unchanged)
d) that's not true, as we know that the number of North American children who are underweight has steadily increased and not declined
e) new information has been introduced (the incidence of obesity ... tends to increase as the children grow older), not encountered in the text

hence by POE we conclude in c. Note, it's not clear from the text whether the number of not obese children increase or decline. As you may know from the elementary math, the increase in numerator and denominator by any positive value may shift the ratio, that is the ratio will be increased. Here it's the same, the fat percentage of the obese children could increase with and without the change in the number of not obese children. We already know that the number of obese children was steadily increasing -that's enough to get >85% change in the fat rate. The only possibility to find the correct answer here is to apply POE method.