I think it's best to review the actual article (which actually is quite heavy on the stats indeed) before we start taking potshots at the simplistic NPR coverage. I'm distracted at the moment, but I'm not actually grasping the original article that well myself; however, it's definitely occurred to me to wonder why we assume that the bell curve is a constant.
Because the
Central Limit Theoren says that for *any* distribution of an underlying variable, for a large enough value of N, a normal distribution will be observed.
I think ultramarina's question is a good one. The Central Limit Theorem doesn't say that the empirical distribution will be normal. It says that with a large enough N, the distribution of the sample means will be approximate a normal distribution around the population mean. It is about determining whether your sample is representative of the population, not that the empirical data itself is normally distributed.
The assumptions regarding the bell curve come from a different source than the central limit theorem. I don't really have time to read the actual journal article, but I am guessing that it is being described inaccurately by the NPR coverage (as is often the case).
