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Glossary

Kelly criterion

The bet-sizing formula that maximizes the long-run growth rate of a bankroll. In tournament poker it determines what fraction of bankroll to risk per buy-in given expected ROI, variance, and bankroll size.

The Kelly criterion is the bet-sizing formula that maximizes the expected logarithm of wealth — equivalently, the long-run geometric growth rate of a bankroll. For a binary bet with probability p of winning a payoff of b-to-1, the Kelly fraction is f* = (bp − q) / b, where q = 1 − p.

For tournament poker the formula generalizes to a multi-outcome distribution (many possible finishes, a payout vector, sometimes bounties). The Kelly stake is no longer closed-form for arbitrary payouts but is computed by maximizing E[log(W + payoff)] over the sale percentage s ∈ [0, 1]. For a single-payout tournament the optimum is closed-form.

Practical note: most pros use fractional Kelly (0.25× to 0.5× full Kelly) because full Kelly is the volatility ceiling — it maximizes growth but produces drawdowns that are emotionally and operationally untenable. The “20 buy-ins” heuristic is a crude proxy for fractional Kelly that under-fits high-ROI players and over-fits low-ROI ones.

In Mucho+MOTA, the Kelly engine reads from the player’s Bayesian skill model posterior and returns per-event sell percentages and growth rates. See the full Kelly essay and the Kelly primer.

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