In this guide
Key takeaway: Within prediction markets, a share's market price functions as the probability estimate. When a YES share trades at $0.65, participants collectively signal a 65% likelihood that the outcome will occur. Grasping this relationship between market valuation and probabilistic assessment forms the cornerstone of successful market participation.
Should you transition from traditional sports wagering, prediction market odds operate on fundamentally different mechanics. Fractional odds (5/1), American odds (+400), and decimal odds (5.0) do not apply here. Instead, prediction markets employ a transparent mechanism: share valuation directly mirrors the market's probability assessment.
Price = Probability
All prediction market contracts present two opposing positions: YES and NO. Prices consistently approach $1.00 in aggregate (accounting for modest spreads retained by liquidity providers). Interpretation follows this pattern:
- YES at $0.72 = Collective assessment: 72% likelihood the event materialises
- NO at $0.28 = Collective assessment: 28% likelihood the event fails to materialise
- YES at $0.50 = Equiprobable outcome — market sentiment remains neutral
- YES at $0.95 = Overwhelming consensus — merely 5% probability of non-occurrence
Calculating Your Expected Value
Expected value (EV) establishes whether a position generates sustainable profitability across repeated transactions. The calculation follows this framework:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Illustration: Suppose "Event X" trades at $0.40 (40% implied), yet your analysis suggests 55% true probability. Purchasing YES at $0.40 yields:
- Gain upon YES resolution: $1.00 - $0.40 = $0.60
- Loss upon NO resolution: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
Positive EV signals an edge-positive position. Accumulating many such trades with positive expectation generates compounding returns.
The Spread
The gap separating the highest purchase offer (bid) from the lowest sale offer (ask) constitutes the spread. Polymarket's active contracts typically exhibit 1-3 cent spreads. This parallels the "vig" familiar to sports bettors, though substantially narrower:
- Prediction market spread: 1-3% (functionally equivalent to vig)
- Sports betting vig: 5-15% embedded within quoted odds
- Probability overround: Prediction markets see YES + NO sum near $1.00. Sports betting typically shows combined implied probabilities of 110-115%
Reading the Order Book
The PolyGram order book depth visualisation displays all unexecuted purchase and sale orders across price tiers. This information reveals:
- Liquidity: Transaction volume achievable without substantial price slippage
- Support/resistance: Valuation thresholds where concentrated orders establish "barriers" limiting price fluctuation
- Market sentiment: Whether buyers or sellers dominate activity at prevailing valuations
Converting to Traditional Odds
Should conventional odds notation feel more intuitive:
| Market Price | Implied Prob. | Decimal Odds | American Odds |
| $0.80 | 80% | 1.25 | -400 |
| $0.65 | 65% | 1.54 | -186 |
| $0.50 | 50% | 2.00 | +100 |
| $0.25 | 25% | 4.00 | +300 |
| $0.10 | 10% | 10.00 | +900 |
Common Mistakes
- Conflating valuation with trade quality: A $0.90 position carries no inherent disadvantage versus a $0.10 position — only whether pricing accurately reflects true probability matters
- Neglecting spread costs: Thinly traded markets may feature 5-10 cent spreads, substantially eroding any statistical advantage
- Excessive conviction: Before assuming the market misprices an outcome, consider why thousands of participants may hold contrary views
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