📚 Stock Market Glossary

The Kelly Criterion is a foundational mathematical formula developed in 1956 by Bell Labs researcher John L. Kelly Jr. to determine the optimal percentage of a bankroll to allocate to a favorable bet or investment.

By calculating the exact trade-off between win probability (p) and payoff odds (b), it maximizes the expected value of the logarithmic wealth growth over time while mathematically eliminating the risk of ruin.

• Preventing Over-Betting & Ruin: Even an edge with 80% win probability will eventually bankrupt a trader if position sizes are too aggressive (e.g., betting 50% per trade), due to inevitable consecutive drawdown streaks.
• Solving Under-Betting Stagnation: Betting too conservatively leaves massive compound growth on the table. Kelly identifies the exact mathematical peak of the capital growth curve.
• Practical Application via Fractional Kelly: Because real-world stock probabilities cannot be estimated with 100% precision, elite quants deploy 'Half-Kelly' (0.5x) or 'Quarter-Kelly' (0.25x) to smooth out drawdowns while capturing 75% to 90% of maximum growth.

Legendary quants like Ed Thorp and Jim Simons built their risk management engines on Kelly principles. If the formula outputs a negative fraction (f* <= 0), it signifies negative expectancy, commanding a strict 'No-Trade' decision.