Price vs Probability
Definition
Price versus probability is the gap between a contract's market price and the true likelihood of the event. Theory shows prices can equal traders' average beliefs under specific conditions; in practice fees, bid-ask spreads, limited liquidity, constraints on how much traders can bet, and risk preferences can move prices away from both average beliefs and actual frequencies.
Why it matters
Treating every price as a probability invites systematic mistakes, especially buying cheap longshots because they look like value. Investors should ask whether the market is liquid, how wide the spread is, how near resolution it is, and whether known biases at that price level apply.
Common misconceptions
- •A market price is not the probability; it is an estimate shaped by who trades and how.
- •Cheap contracts are not underpriced by default; research often finds the opposite.
- •Accuracy in liquid headline markets does not carry over to thin markets.
- •A null result in a small sample does not show a bias is absent.
Technical details
Forces that separate price from probability
Fees and spreads: a wedge that is largest relative to price for cheap contracts.
Liquidity: thin books let a few orders move the price.
Wealth and position constraints: limit how much informed traders can correct a price.
Risk preferences: affect how aggressively traders with different beliefs trade.
Time to resolution: capital tied up for months has an opportunity cost.
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- Why might a prediction market price be a poor probability estimate?
- Several forces. Wolfers and Zitzewitz show that when traders with low risk aversion trade aggressively near 50-50 and 'investors on the short side of a longshot are more likely to be wealth-constrained,' prices are 'biased toward one half.' If average beliefs are accurate, this implies 'a favorite-longshot bias, in which longshots win less often than might be expected given the prediction market price.' Fees and spreads add a further wedge, largest relative to price for cheap contracts.
- Are prediction markets accurate?
- Often useful, sometimes better than alternatives. The CFTC says that when predictions are combined, 'prediction markets can sometimes forecast event outcomes better than polling or other forms of forecasting.' Accuracy varies by market, liquidity and time to resolution.
- What did AltStreet's own data show?
- In AltStreet's Kalshi study, none of nine series covering 195 settled contracts showed statistically significant calibration error after restricting to quotes captured within three hours of settlement and spreads of 10 cents or less. The study notes this is a small, liquid subsample with low statistical power, and that a larger academic study of more than 300,000 Kalshi contracts reports low-price contracts winning less often than needed to break even after fees.