In this guide
Key takeaway: Within prediction markets, a share's price functions as the probability itself. When a YES share trades at $0.65, the collective market assessment reflects a 65% likelihood of that outcome occurring. Grasping this fundamental relationship between price and probability forms the cornerstone of successful market participation.
For those transitioning from traditional sports wagering, prediction market odds operate on an entirely different framework. Fractional odds (5/1), American odds (+400), and decimal odds (5.0) do not apply here. Instead, prediction markets employ a more transparent mechanism: share prices function as direct probability indicators.
Price = Probability
All prediction market contracts split into two opposing positions: YES and NO. Their prices consistently total approximately $1.00 (accounting for a modest spread retained by the market maker). Interpreting them follows this pattern:
- YES at $0.72 = Collective market view suggests a 72% likelihood of occurrence
- NO at $0.28 = Collective market view suggests a 28% likelihood of non-occurrence
- YES at $0.50 = Evenly balanced — market participants show no clear bias
- YES at $0.95 = Overwhelming consensus — merely a 5% possibility of failure
Calculating Your Expected Value
Expected value (EV) serves as the metric determining whether a position generates profit over time. The calculation follows this straightforward approach:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Illustration: Suppose "Event X" trades at $0.40 (40%), yet your analysis suggests the genuine probability stands at 55%. Should you acquire YES at $0.40:
- Gain if YES materialises: $1.00 - $0.40 = $0.60
- Loss if NO materialises: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
When EV turns positive, that position yields profitable returns statistically. Across numerous transactions, accumulated positive EV generates tangible wealth accumulation.
The Spread
The gap separating the highest purchase offer (best bid) from the lowest sale offer (best ask) constitutes the spread. Polymarket's actively traded contracts typically exhibit spreads ranging from 1-3 cents. This resembles the "vig" present in sports betting, though substantially narrower:
- 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 prices converging near $1.00. Sports betting typically shows implied probabilities summing to 110-115%
Reading the Order Book
The PolyGram order book depth display presents all unexecuted purchase and sale orders across various price points. This information reveals:
- Liquidity: The quantity available for purchase or sale without substantially shifting the price
- Support/resistance: Price zones where substantial orders accumulate, forming barriers that impede price swings
- Market sentiment: Whether buying pressure outweighs selling pressure, or vice versa, at prevailing prices
Converting to Traditional Odds
Should you prefer working with 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
- Mistaking price for bet quality: A $0.90 position carries no inherent disadvantage versus a $0.10 position — what determines value is whether pricing accurately captures genuine probability
- Overlooking the spread: Thinly traded markets frequently show spreads of 5-10 cents, which significantly erodes your profit margin
- Excessive confidence: Should you believe the market misprices an outcome, consider why thousands of participants may hold the opposite view
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