Chapter 6 — Glossary: xG, Glicko-2, Brier, edge, Kelly and every key term

Probability that each chance becomes a goal, based on shot location, angle, body part, defensive pressure. A long-range shot is worth 0.03 xG, a penalty 0.78. Sum match xG and you get how many goals the team 'deserved' to score. Long-term, goals converge to xG: if a team scores much less, it's statistically unlucky.

*Glicko-2 is a rating like ELO but with two additions: a deviation (RD = how sure we are of the rating) and a volatility. A newly-promoted team has a high rating but huge RD = the model knows it doesn't know. Brier score* measures how far predicted probabilities are from real outcomes. 0 = perfect, 1 = terrible. Under 0.25 for 1X2 is great. PronoStats sits around 0.79 raw, 0.64 after Groq correction.

*Edge = (your probability × odds) - 1. Measures how much better your estimate is than the bookmaker's. Positive edge = potential value bet. EV* is the monetary expectation per unit staked: stake × edge. Example: stake €10, edge 10% → EV +€1. Doesn't mean you win €1 tonight, it means IF you play 1000 identical bets, you average +€1000. Single bets can lose; the portfolio wins over time.

Formula to size your stake: f = (b·p - q) / b, where b = odds - 1, p = your probability, q = 1 - p. Returns the optimal fraction of bankroll to risk. Mathematically maximizes geometric growth rate. *Problem: psychologically brutal (in a losing streak you can lose 60% in 10 bets). Solution: ½ Kelly (calmer) or ¼ Kelly* (much calmer). Pros use ¼.

*ROI (Return On Investment) = profit / capital invested. A season with +€200 on €2000 staked = +10% ROI. Yield is the same for bettors = profit / stake volume. Good range = +3-7% long-term (above 10% = suspect or small sample). Drawdown* = max loss from peak to next trough. Tracking drawdown is even more important than ROI: it tells you if your strategy lets you survive to the end.

*1X2 = home / draw / away. BTTS (Both Teams To Score) = do both teams score? Yes/No. DNB (Draw No Bet) = draw refunds. AH (Asian Handicap) = handicap with fractions that eliminate the draw. Over/Under = total goals above/below a line (most common: 2.5). Push* = when the outcome lands exactly on the handicap or line (e.g. 1-1 on AH 0) and the bet refunds the stake — no win, no loss.