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Glossary

Variance

The dispersion of tournament outcomes around expected value. In tournament poker variance is heavy-tailed and dominates short-run results — most observed deviation from expected ROI is variance, not skill change.

Variance in tournament poker is the dispersion of outcomes around expected value, typically expressed as the standard deviation of ROI per tournament (in buy-ins) or as the per-tournament second moment of net profit.

Tournament variance is heavy-tailed: a small fraction of outcomes (deep runs, final tables, wins) account for most of the lifetime EV. This is why standard normal-distribution intuition (68% within 1σ, etc.) understates the chance of long, painful downswings. A player with 30% ROI and 2.5 buy-in standard deviation can have 200-tournament stretches where realized ROI is negative — and this is statistically normal, not a leak.

Two practical implications:

  1. Bankroll requirements are set by variance, not ROI. Two players with identical 25% ROI but variance 2.0 vs 3.5 buy-ins need very different bankrolls to play the same stake.
  2. Skill-vs-variance attribution is hard. Most disagreements about whether a player is “running bad” or “playing bad” are unresolvable in the short run because the variance signal swamps the skill signal at typical sample sizes. The Bayesian skill model addresses this by partial pooling.

See the variance learn primer, embracing disaster, and sample size intuition.

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