A price falling by 20% does not mean a team's chance of winning has risen by 20 percentage points. Decimal odds and implied probabilities use different scales. Describing both as an “odds change percentage” can conceal which quantity was actually measured.
Consider two pre-match quotes for the same home-win selection at the same bookmaker: 2.50 at the first observation and 2.00 at the second. The numbers below are a worked example, not prices from an actual fixture. Keeping the market and settlement terms fixed is part of the example.
Calculate the price change first
Subtract the initial price from the later price, then divide by the initial price: (2.00 − 2.50) / 2.50 = −0.20. The decimal price has fallen by 20%. Notice that the starting value is the denominator. Reversing the move, from 2.00 to 2.50, produces a 25% increase rather than a 20% increase.
Now convert each price to raw implied probability using 1 / decimal odds. The first quote corresponds to 40%; the second corresponds to 50%. The difference is 10 percentage points. Relative to the original 40%, the implied probability has increased by 25%.
| Measurement | Change |
|---|---|
| Decimal price | −20% |
| Raw implied probability | +10 percentage points |
| Relative implied probability | +25% |
Do not turn a conversion into a forecast
The resulting 50% is a conversion of an offered price. It is not an independently estimated win probability. A bookmaker's margin and pricing decisions remain embedded in the quote. To examine changes across the complete three-way market, retain the draw and away prices at both observation times as well.
For instance, a change in the overall market margin can alter the relationship between a raw implied probability and a margin-adjusted estimate. Watching just the home selection leaves that distinction unresolved. There is no calculation in this example that establishes the team's true chance of winning.
Write a finding another reader can check
A useful research note identifies the fixture, bookmaker, selection, market rules and both timestamps with time zones. “The home price moved from 2.50 to 2.00 between 14:00 and 16:00 UTC; raw implied probability rose by 10 percentage points” is specific about the observation.
Also record whether each price was pre-match or in-play. A quote collected after kick-off can reflect a new score and a different amount of time remaining. It should not quietly extend a pre-match price series. Likewise, a first-half market is a different outcome from a full-time market.
If team news appeared between the observations, record its publication time separately. The coincidence supplies context, but the two prices alone cannot prove that the announcement caused the move. Clear units and a defensible comparison are more useful than a dramatic percentage headline.
Sources and method
Calculations and scenarios are original teaching examples, not observed match data, product performance or predictions. Sources support the stated concepts and methods. This article was prepared with AI assistance; sources and calculations were checked.
- Smarkets: implied probability formulas — accessed 14 September 2026