Expected value answers one question: if you placed this exact bet a thousand times, would you end up ahead? It is calculated as (probability of winning × decimal odds) − 1, expressed per unit staked. Positive EV (+EV) means the price beats the true probability; negative EV means the bookmaker's margin or a mispriced view is working against you.
EV is a long-run concept. A +EV bet can lose many times in a row, and a −EV bet can land — variance dominates small samples. The discipline of profitable betting is refusing to judge decisions by single results and judging them by the price you took instead. Over hundreds of bets, results converge toward expectation.
You estimate a draw at 30% probability and find odds of 3.75. EV = (0.30 × 3.75) − 1 = +0.125 — an expected 12.5p profit per £1 staked. If your probability estimate is right, betting this price repeatedly is profitable even though it loses 70% of the time.