Odds and probabilities
Decimal odds. The European format: what you receive for €1 staked, stake included. Odds of 2.35 return €2.35 for €1, i.e. €1.35 of net profit.
Fractional odds. The British format: 27/20 means 27 won for 20 staked (decimal 2.35).
American odds. +135: win 135 for 100 staked; −150: stake 150 to win 100. See the converter.
Implied probability. The inverse of the decimal odds: odds of 2.00 "imply" 50%. It includes the bookmaker's margin, so it always overstates the real chance. Guide.
True probability (or fair probability). The best estimate of the chance that an outcome happens, obtained from the reference bookmakers' odds once their margin is removed. It is the "chance of winning" shown on every Proba Bet bet.
Fair odds. The inverse of the true probability: the odds a bookmaker with no margin would offer.
Margin (overround, vig, juice). The excess of the sum of implied probabilities above 100%. It is the bookmaker's revenue and the bettor's cost: from 2% (Pinnacle) to 10% (exotic markets). Guide, calculator.
Devigging. Removing the margin from the odds to recover the true probabilities. Proportional, power and Shin methods.
Favourite-longshot bias. Bookmakers' tendency to load more margin onto underdogs than onto favourites. The finer devigging methods correct it.
The edge
Value bet. A bet whose odds exceed the fair odds. Edge = probability × odds − 1. Guide.
Edge (EV). The expected profit per euro staked. +5% means 5 cents of average profit per euro, over the long run.
Expected edge. The share of the raw edge that survives to the close, measured by Babeth over thousands of bets; the stake is calculated on it.
Selection (grade A, B, C). On Proba Bet, every detected bet gets a grade: A = edge ≥ 2% at a reliable bookmaker, in a reliable league, confirmed over two scans; B and C = reservations (stale odds, thin market, unreliable league…).
Steam. A fast, marked move in the odds at the reference bookmakers, the sign that a large informed volume has just come in. A bet "with the steam" is more likely to keep its edge.
Drift. The opposite move: the odds lengthen, the probability falls.
The proof
Closing line. The last reference odds before kick-off; the most informed probability in existence. Guide.
CLV (closing line value). Odds taken ÷ fair closing odds − 1. When positive, it proves you are getting good prices, whatever the result.
Beating the close. Taking odds better than the fair closing odds. The share of bets that beat the close is the most reliable measure of a real edge.
Track record. The public history of every flagged bet, settled with the official score, with its closing line. Ours is here.
Brier score. The accuracy measure of a probability: the mean squared error between the stated probability and the result (0 = perfect, 0.25 = a coin flip on a two-way market).
Calibration. A probability is calibrated if, among all the bets stated at 60%, about 60% win. Babeth measures and corrects hers on every outcome she publishes.
The stake
Bankroll. The sum set aside for betting, defined once and for all. Guide.
Unit. 1% of the bankroll; the scale in which stakes are expressed.
Kelly criterion. The fraction of the bankroll that maximizes long-term growth: (p × c − 1) / (c − 1). Guide, calculator.
Fractional Kelly. A fraction (0.25 to 0.5) of full Kelly, to cushion the uncertainty on the probability. Proba Bet also caps every stake at 5% of the bankroll.
Variance. The normal swings of results around the expectation. A bettor with a real edge goes through drawdowns of 20 to 30%; the simulator shows them.
Drawdown. The gap between a bankroll peak and the trough that follows, as a percentage.
The market
Sharp bookmaker. A low-margin, high-limit operator that accepts winners and whose odds serve as the benchmark (Pinnacle). Guide.
Soft bookmaker. A mainstream, high-margin operator that limits winners and whose odds follow the market with a lag — where value bets are born.
Exchange (betting exchange). A platform where bettors bet against each other (back / lay); the price is set by supply and demand. Betfair Exchange is the most liquid in the world.
Back / lay. On an exchange, "back" = bet on an outcome (as at a bookmaker); "lay" = bet against it (play the bookmaker).
Liquidity (matched). The amount traded on an exchange market. The higher it is, the more reliable the price.
Surebet (arbitrage). A combination of odds at different bookmakers that covers every outcome with a guaranteed profit. Guide, calculator.
Parlay (accumulator). A ticket grouping several bets: every selection must win. The odds multiply, and so do the margins. Guide.
Account limiting. The cut in maximum stakes a bookmaker imposes on a bettor it considers a winner. The reason surebets and value bets are played with discretion.
Handicap (spread). A market where points are added to or taken from a team before comparing the scores (Lyon −1.5).
Total (over / under). A market on the total number of points or goals, above or below a line (over 2.5 goals).
1X2 (moneyline). The match-winner market: home (1), draw (X), away (2). Without the draw, it is called the moneyline or "match winner".