Kelly Criterion
Definition
The Kelly Criterion is a formula that helps determine the optimal fraction of capital to allocate to a given investment or bet in order to maximize long-term portfolio growth. It balances the probability of success with the size of potential gains and losses.
Why It Matters to Investors
- Maximizes geometric (compounded) growth over time
- Helps avoid overbetting, which can lead to ruin
- Provides a rational basis for position sizing
- Applies to both single bets and diversified portfolios
- Requires accurate inputs to be effective
The TiltFolio View
The Kelly Criterion is theoretically elegant, but practically fragile. Its effectiveness depends entirely on accurate estimates of probabilities and returns, a notoriously difficult challenge in real-world investing.
Neither TiltFolio system uses Kelly sizing in their approaches. While the framework is intellectually appealing, even small errors in estimating probabilities can lead to oversized risk. Instead, both systems use more conservative and robust approaches to capital allocation that prioritize drawdown control and long-term consistency over theoretical optimality.
TiltFolio Adaptive uses trend-following signals to determine allocation, while TiltFolio Balanced maintains consistent diversified allocation. Both systems recognize that volatility and correlation across asset classes can shift rapidly, something the pure Kelly model does not adapt to well.
Real-World Application
• Used by gamblers and quantitative traders to size bets with a known edge
• Applied in high-frequency trading systems to balance risk and reward
• Sometimes used in portfolio management to guide position sizing based on expected returns