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 believe the outcome has a 65% likelihood. Grasping this relationship between price and probability forms the cornerstone of successful market participation.
Coming from a sports-betting background, prediction market odds operate on entirely different mechanics. You won't encounter fractional odds (5/1), American odds (+400), or decimal odds (5.0). Instead, prediction markets employ a straightforward approach: share prices function as direct probability indicators.
Price = Probability
Each prediction market contract splits into two opposing positions: YES and NO. These prices generally sum to roughly $1.00 (with a modest spread retained by the exchange). The interpretation works as follows:
- YES at $0.72 = Collective market view suggests 72% likelihood the outcome materialises
- NO at $0.28 = Collective market view suggests 28% likelihood the outcome does not materialise
- YES at $0.50 = Perfectly balanced — the market shows no lean either direction
- YES at $0.95 = Overwhelming consensus — merely 5% probability of the opposite outcome
Calculating Your Expected Value
Expected value (EV) establishes whether a position generates profit over time. The calculation follows this framework:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Suppose a market quotes "Event X" at $0.40 (40% implied), yet your analysis suggests the genuine probability sits at 55%. Purchasing YES at $0.40 yields:
- Upside if YES wins: $1.00 - $0.40 = $0.60
- Downside if NO wins: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
Positive EV signals the position should yield profit statistically. Across numerous positions, positive EV accumulates into measurable gains.
The Spread
The gap separating the highest willing buyer (bid) from the lowest willing seller (ask) constitutes the spread. On Polymarket, active markets typically show spreads between 1-3 cents. This mirrors the "vig" from sports betting yet proves substantially tighter:
- Prediction market spread: 1-3% (equivalent to vig)
- Sports betting vig: 5-15% embedded within quoted odds
- Implied overround: Prediction markets see YES + NO sum near $1.00. Sports betting sees implied probabilities frequently total 110-115%
Reading the Order Book
The PolyGram order book depth chart displays all unexecuted buy and sell orders across price tiers. This reveals:
- Liquidity: The quantity available for purchase or sale without shifting the price substantially
- Support/resistance: Price zones containing concentrated orders, forming "walls" that slow price shifts
- Market sentiment: Whether buying or selling dominates at present valuations
Converting to Traditional Odds
Should traditional 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
- Treating price as a quality indicator: A $0.90 share carries no inherent advantage or disadvantage versus a $0.10 share — accuracy depends on whether the price aligns with actual probability
- Overlooking the spread: Less liquid markets may feature 5-10 cent spreads, which substantially erodes your statistical edge
- Excessive conviction: Before betting against the consensus, consider why thousands of participants hold a different view than yours
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