Variance is a statistical measure of how widely individual outcomes deviate from the expected value. In gambling, it determines the magnitude of short-term swings a player experiences, even when the long-term mathematical expectation remains constant.

Two casino games can share the same house edge yet produce vastly different player experiences due to variance. A low-volatility slot machine with a 96% RTP might deliver frequent small payouts, keeping the player's balance relatively stable across a session. A high-volatility slot with the same RTP concentrates its returns into rare large payouts, meaning most sessions end in loss while occasional sessions produce outsized wins. The RTP is identical in both cases; only the distribution of outcomes differs.

In poker, variance manifests as extended winning and losing streaks that can span thousands of hands. A skilled cash game player with a win rate of 5 big blinds per 100 hands might experience a downswing of 30 or more buy-ins that lasts tens of thousands of hands — a statistically normal occurrence that feels devastating in practice. Understanding this distinction between short-term results and long-term expectation is essential for maintaining discipline and proper bankroll sizing.

Standard deviation, expressed in units relevant to the game, quantifies volatility numerically. For slot machines, this is often measured per spin relative to the bet size. For poker, it is measured in big blinds per 100 hands. Players can use session simulation tools to model likely outcome distributions over a chosen number of sessions, helping to set realistic expectations about the range of results they may encounter.