[flight2q]

Say a few weeks ago you sat down at a table with Graycat HU, and after the session you asked yourself did that guy see my holecards. The a priori distribution is whatever probability you thought he had of seeing your hole cards before you sat down with him. 1 in a million? 1 in a billion? or whatever.

Then you analyze the session to adjust that a priori probability based on the data. But in doing so, you need to realize that your data is biased, because you don't do this analysis after every session. But if you sit at a table with Graycat right now and play until he quits you (or play until a prescribed number of hands, and throw out the sample if he quits you earlier), and analyze based on those results, good or bad, then you won't have selection bias.