In this guide
Key takeaway: Within prediction markets, a share's market price functions as the probability itself. When a YES share trades at $0.65, the collective market assessment reflects a 65% likelihood that the outcome will occur. Grasping this fundamental relationship between valuation and likelihood forms the bedrock of successful market participation.
Those transitioning from traditional sports betting will notice that prediction market odds operate on entirely different mechanics. You won't encounter fractional notation (5/1), American-style formatting (+400), or decimal conventions (5.0). Instead, prediction markets employ a more transparent framework: the share's market value directly encodes the probability the market assigns to an outcome.
Price = Probability
All prediction market contracts split into two opposing positions: YES and NO. Their prices consistently total roughly $1.00 (accounting for a modest spread retained by liquidity providers). The interpretation works as follows:
- YES at $0.72 = Collective market view suggests 72% likelihood of occurrence
- NO at $0.28 = Collective market view suggests 28% likelihood of non-occurrence
- YES at $0.50 = Evenly balanced — market shows no lean toward either direction
- YES at $0.95 = Overwhelming consensus — merely 5% probability assigned to the contrary outcome
Calculating Your Expected Value
Expected value (EV) serves as the metric determining whether a position generates profit across repeated trades. The underlying calculation remains straightforward:
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 the genuine likelihood stands at 55%. Should you acquire YES at $0.40:
- Upside if YES resolves: $1.00 - $0.40 = $0.60
- Downside if NO resolves: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
Whenever EV turns positive, the position carries favourable expected returns. Across numerous such positions, this mathematical edge translates into measurable wealth accumulation.
The Spread
The gap separating the highest willing buyer price from the lowest willing seller price constitutes the spread. On Polymarket, actively traded contracts typically show spreads ranging 1-3 cents. This parallels the "vig" concept familiar to sports bettors, though substantially tighter:
- Prediction market spread: 1-3% (functionally equivalent to vig)
- Sports betting vig: 5-15% embedded within the quoted odds
- Implied overround: Prediction markets see YES + NO sum near $1.00. Sports betting typically inflates this to 110-115%
Reading the Order Book
The PolyGram order book depth display reveals all outstanding bids and asks stacked at various price tiers. This visibility provides:
- Liquidity: The volume available for purchase or sale without substantial price slippage
- Support/resistance: Price zones where concentrated orders form barriers, dampening directional movement
- Market sentiment: Whether accumulated interest skews toward accumulation or distribution at prevailing levels
Converting to Traditional Odds
Should you prefer conventional odds notation:
| 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
- Treating price as a quality indicator: A $0.90 contract holds no inherent advantage over a $0.10 contract — only whether the quoted price aligns with true likelihood matters
- Overlooking the spread: Thin markets can display spreads of 5-10 cents, substantially eroding your mathematical advantage
- Excessive conviction: Before wagering against the consensus, ensure you can articulate why thousands of participants have reached a different conclusion
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