Understanding Prediction Market Odds and Probability

Key takeaway: Within prediction markets, share price functions as a direct expression of probability. When a YES share trades at $0.65, the collective market assessment indicates a 65% likelihood of that outcome materialising. Grasping this fundamental relationship between price and probability forms the cornerstone of successful market participation.

Those transitioning from traditional sports betting will notice that prediction market odds operate quite differently. You won't encounter fractional notation (5/1), American-style formats (+400), or decimal representations (5.0). Instead, prediction markets employ a more straightforward approach: share prices function as direct proxies for implied probability.

Price = Probability

All prediction market contracts feature two opposing positions: YES and NO. These prices typically sum to roughly $1.00 (accounting for a modest spread retained by the market operator). The interpretation follows this pattern:

YES at $0.72 = Aggregate market view suggests 72% probability of occurrence
NO at $0.28 = Aggregate market view suggests 28% probability of non-occurrence
YES at $0.50 = Even odds — the market shows no clear lean either direction
YES at $0.95 = Approaching certainty — merely a 5% probability of the opposite outcome

Calculating Your Expected Value

Expected value (EV) serves as the metric for assessing whether a position generates long-term profitability. The calculation employs this straightforward formula:

EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)

Illustration: Suppose "Event X" trades at $0.40 (40% implied), yet your own assessment puts true probability at 55%. Should you acquire YES at $0.40:

Gain if YES resolves: $1.00 - $0.40 = $0.60
Loss if NO resolves: $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 expectancy-profitable position. Across numerous trades, such positive expectations accumulate into tangible wealth creation.

The Spread

The gap separating the highest willing buyer price (bid) from the lowest willing seller price (ask) constitutes the spread. Polymarket's more active contracts typically exhibit spreads ranging from 1-3 cents. This mechanism parallels the "vig" found in sports betting, though substantially tighter:

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 prices converge near $1.00. Sports betting typically exhibits implied probabilities totalling 110-115%

Reading the Order Book

The PolyGram order book depth display presents all unexecuted buy and sell orders arranged by price tier. This information reveals:

Liquidity: The magnitude of volume available for purchase or sale without substantially shifting prevailing prices
Support/resistance: Price zones where substantial orders congregate, forming barriers that inhibit price swings
Market sentiment: Whether buyers or sellers maintain greater urgency at the present price level

Converting to Traditional Odds

Should you prefer working within established odds frameworks:

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 — only whether the quoted price accurately reflects genuine probability matters
Overlooking spread costs: In less-traded markets, spreads can widen to 5-10 cents, substantially eroding your statistical edge
Excessive confidence in contrarian views: When you believe the market has mispriced something, consider carefully why thousands of other participants hold the opposite conviction

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