In this guide
Key takeaway: The Kelly Criterion determines what percentage of your capital to allocate to each wager, accounting for your informational advantage and available odds. Within prediction markets, this framework guards against the two cardinal errors: deploying excessive capital (risking total loss) and deploying insufficient capital (forgoing available returns).
The distinction between sustained profitability and financial collapse hinges on position sizing discipline. The Kelly Criterion — a mathematical framework introduced by John Kelly, a researcher at Bell Labs, in 1956 — establishes the theoretically optimal stake magnitude for achieving maximum compound growth rates. This article explores its implementation within prediction market contexts.
The Kelly formula
For a two-outcome prediction market (YES/NO), the Kelly fraction resolves to:
f* = (p * b - q) / b
Where:
- f* = percentage of capital to deploy
- p = your assessed likelihood of success
- q = likelihood of failure (1 - p)
- b = decimal odds (return / investment). For a prediction market contract trading at price c, b = (1 - c) / c
Worked example
Suppose you assess a 60% probability that an outcome materialises as YES. Current market valuation stands at 45 cents (suggesting 45% implied probability).
- p = 0.60, q = 0.40
- b = (1 - 0.45) / 0.45 = 1.222
- f* = (0.60 * 1.222 - 0.40) / 1.222 = (0.733 - 0.40) / 1.222 = 0.272
The Kelly formula recommends committing 27.2% of your capital. If your total capital is $1,000, this translates to a $272 position.
Why full Kelly is dangerous
The Kelly formula presumes you possess exact knowledge of your true probability — an assumption that rarely holds in practice. Miscalculating your informational edge results in severe overexposure. Seasoned market participants consistently employ fractional Kelly strategies:
- Half Kelly (f*/2): The industry standard. Surrenders roughly 25% of theoretical growth but diminishes drawdowns by half
- Quarter Kelly (f*/4): Risk-averse methodology suited to situations where edge confidence is limited
- Capped Kelly: Establish a ceiling of 5-10% per individual market, overriding Kelly calculations if they exceed this threshold
Applying Kelly to multi-market portfolios
Operating across numerous prediction markets concurrently demands recalibration of individual Kelly allocations. The aggregate of all Kelly percentages must remain at or below 1.0 (your entire bankroll). Operationally, restrict cumulative deployment to 50% or less, preserving capital for emerging opportunities and comparing odds across platforms.
When Kelly does not apply
Kelly's validity depends on reliable probability estimation. Several scenarios undermine this assumption:
- Unprecedented events lacking historical data for calibration
- Interdependent markets (presidential election results and legislative majorities exhibit correlation)
- Situations where your analysis provides no advantage relative to prevailing market pricing
PolyGram provides an integrated Kelly Criterion calculator to optimise position sizing before execution. The analytical suite encompasses payoff visualisations and maximum drawdown metrics. Start trading on PolyGram →