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 is that a 65% likelihood exists for that outcome to occur. Grasping this fundamental relationship between price and probability forms the cornerstone of successful market participation.
Coming from a sports betting background, prediction market odds operate quite differently. Fractional odds (5/1), American odds (+400), and decimal odds (5.0) do not exist here. Instead, prediction markets employ a more straightforward approach: share prices function as direct probability indicators.
Price = Probability
All prediction market contracts feature two opposing sides: YES and NO. These prices consistently total roughly $1.00 (accounting for a modest spread retained by the market operator). Here is what these prices convey:
- YES at $0.72 = The market assesses a 72% likelihood of the outcome materialising
- NO at $0.28 = The market assesses a 28% likelihood the outcome does not materialise
- YES at $0.50 = An even split — the market holds no clear bias either direction
- YES at $0.95 = Overwhelming consensus — merely a 5% probability of the opposite occurring
Calculating Your Expected Value
Expected value (EV) establishes whether a position generates profit across repeated trades. The calculation follows this straightforward formula:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Illustration: A market quotes "Event X" at $0.40 (40% implied), yet you assess the genuine probability at 55%. Purchasing YES at $0.40 yields:
- 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, the position carries favourable expected returns. Across numerous trades, this positive expectation accumulates into tangible wealth creation.
The Spread
The gap separating the highest purchase offer (best bid) from the lowest sale offer (best ask) constitutes the spread. On Polymarket, heavily-traded contracts typically exhibit spreads of 1-3 cents. This mirrors the "vig" familiar to sports bettors, though substantially tighter:
- Prediction market spread: 1-3% (comparable to vig)
- Sports betting vig: 5-15% embedded within quoted odds
- Implied overround: Prediction markets see YES + NO prices converge near $1.00. Sports betting displays implied probabilities summing to 110-115%
Reading the Order Book
The PolyGram order book depth display presents all outstanding buy and sell orders across price tiers. This reveals:
- Liquidity: The volume available for purchase or sale without shifting the quoted price
- Support/resistance: Price zones where substantial orders congregate, forming "walls" that impede price shifts
- Market sentiment: Whether buying or selling momentum dominates 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
- Treating price as an indicator of trade quality: A $0.90 contract carries no inherent advantage or disadvantage versus a $0.10 contract — the crucial factor is alignment between price and actual probability
- Overlooking the spread: Thinly-traded markets often feature spreads of 5-10 cents, which can erode your potential advantage significantly
- Excessive conviction: Before betting against the consensus, consider whether your reasoning justifies disagreeing with thousands of market participants
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