>A few years ago, a young pro named Phil Galfond published a crucial refinement to Mr. Sklansky's point. He showed that the right way to analyze a poker decision is to consider your opponent's "range"—that is, the full set of different hands that he could plausibly have, given all the actions that he has thus far taken. I find it impossible to believe that concept of a "range" was (a) invented a few years ago and (b)…
On a shameless self promotional note: If there's any online casual players out there and they want to know their opponents range, check out a lil web app I built: http://www.pokerstoker.com. Keeps track of opponents pre-flop play, which is essentially their range.
I actually want to add a little feature that displays how your (previous) hand stacks up against various ranges across the streets. I'll add it when the poker bug bites me again. :-)