> I agree that the above is ambiguous if you really want to press > the point. It uses mu and sigma which describe populations > It does not state whether the population is the class of 2005, > or something wider that would be more useful for generalization. >
That's not the issue. Take any finite population with a data generating process behind it. Population mean is an unbiased estimate of data generating process distribution as Ray pointed out. But once you start getting samples from that population your random error turns into a systematic error (a bias).