[AaronBrown]

Expected value is only as good as the assumptions that underlie it. It's a very reliable guide when your hand depends on what cards will be dealt, it can be treacherous when you hand depends on what other players hold.

For example, if you have a flush draw on the river, there are 9 of 46 cards that will make your flush. But 2 of the 9 (in most cases) will also pair the board and maybe fill a full house; or maybe someone else has a higher flush draw. You also don't know if a bet will be called on the river if you fill your flush.

Another issue is this decision leaves out the chance that someone else is bluffing and will fold even to your missed flush, or that you can win by bluffing if you don't fill. It doesn't consider the effect of your actions now on future hands. And this is the clearest situation for EV, on the river with a drawing hand. Figuring EV preflop with AJ offsuit is even more slippery.

Still, EV is an essential concept for discipline. There's so much randomness in poker that you have to concentrate on EV. If you make a series of 10 even money bets with 60% chances of winning, you have to be happy with your play, even if you lose money (as you often will). If you make a 40% even money bet and win, you have to change your play. If you ignore EV, you'll end up overreacting to the randomness, and making too many decisions based on fear and greed.