Comment by jsprogrammer

Of course the selection of data is arbitrary -- but Rich gives us a definition, which he makes abundantly clear and uses consistently. All definitions can be considered arbitrary. He's not making any claim that we have all the relevant bits of data or that we can be sure what the data really means or represents.

But we can expound on this problem in general. In any experiment where we gather data, how can we be sure we have collected a sufficient quantity to justify conclusions (and even if we are using statistical methods that our underlying assumptions are indeed consistent with reality) and that we have accrued all the necessary components? What you're really getting at is an __epistemological__ problem.

My school of thought is that the only way to proceed is to do our best with the data we have. We'll make mistakes, but that's better than the alternative (not extrapolating on data at all.)