Two bets can share a house edge and feel nothing alike. On a European roulette wheel a straight-up number and a red bet both cost 2.70%, but one wins 2.7% of the time at 35:1 and the other wins 48.6% of the time at even money.
That difference is variance, and it is the only thing bet selection buys you in a game with a flat edge. Wide coverage means small frequent swings; narrow coverage means rare large ones. Neither is cheaper.
Variance is why short results tell you almost nothing. A player can beat a 5.26% American roulette table for an evening without the edge having moved a hundredth of a percent, and a correct blackjack player can lose for a week. The edge is a property of the bet; the outcome is a sample.
Practically, high variance demands a bigger bankroll for the same expected loss. If you want a long session on a fixed budget, bet wide. If you want a chance at a large result, bet narrow, and expect to reach it rarely.
Where the numbers come from
Related terms in odds & money
- House edge — The average share of each bet the casino expects to keep over the long run.
- RTP — Return to player: the mirror of house edge, quoted as what comes back rather than what is kept.
- Expected value — The average result of a bet if it could be repeated indefinitely.
- Standard deviation — A measure of how far results typically stray from the average.
- Bankroll — The money set aside for gambling, kept separate from money needed for anything else.
- Unit — One standard bet, used as the yardstick for measuring results and bankroll.
Where an entry quotes a number, that figure comes from the computed sections of this site rather than from another glossary, so the definition and the derivation cannot drift apart. Each entry links to the page where the number is worked out.