The distribution of group sizes is not normal. It looks like a Normal Distribution that has been pushed to the left, with a long tail to the right. So the 68% rule for plus or minus one standard deviation isn't quite right, but it's also not far off. Also, there is no Central Limit Theorem for any measure of dispersion (range, SD, group size), so collections of data do not tend toward normality.
denton,
Could you elaborate on your reasoning here?
