Expected Value (EV)

Expected value is the long-run average profit or loss of a bet, calculated as the true probability of winning multiplied by the payout, minus the probability of losing multiplied by the stake. A positive-EV bet makes money over a large sample even when it loses often; a negative-EV bet loses over time even when it wins frequently. Every bookmaker market carries a built-in margin, so beating it requires a genuinely better probability estimate than the market's.