As, I understand it, your win rate depends on a few different things. Like, type of count (level I,II, unbalanced), # of index plays, bet spread, house rules, number of decks, penetration, and bankroll.
Qfit has a good calculator for win rate found here. (Dead link: http://www.bjstats.com/bjbr.asp) _Win Rate_
EV or expected value is the sum of the probability of each possible outcome of the experiment multiplied by the outcome value (or payoff). It represents the average amount one "expects" as the outcome of the random trial when identical odds are repeated many times. Note that the value itself may not be expected in the general sense - the "expected value" itself may be unlikely or even impossible.
The above was taken from
wikipedia, and the full article has some pretty good examples. When people say +EV *I think* they are actually talking about win rate. EV and win rate are linked and are highest with optimal betting. I believe you can use CVCX to tweak your bet spread based on the game and your count/bankroll.
The reason you double down on doubling hands is the hand has a +EV and you want to get as much money out as possible during +EV situations. EV will change based on your cards, dealers up card, and the count. BJmath has a chart with the
numbers (Archive copy). Making a -EV game into a +EV game is done by optimal bet sizing, BS + index plays.
This link has some info from
professional blackjack.
I hope all this helps to clarify things for you. If I'm wrong on any of this feel free to correct me.
Josh
