Implied probability
Implied probability turns a price into the percentage chance it implies. For decimal odds, it is one divided by the price.
Decimal 2.00 implies 50%, 4.00 implies 25% and 1.25 implies 80%. The shorter the price, the likelier the outcome in the price's own terms. That figure describes the price and carries the bookmaker's margin inside it, so it is a reading of the market's pricing, never a forecast of the result.
Other formats reach the same figure by other arithmetic. Fractional 6/4 implies 4 ÷ (6 + 4), which is 40%. American +150 implies 100 ÷ (150 + 100), also 40%, while American -200 implies 200 ÷ (200 + 100), or 66.67%. Converting to decimal first and dividing one by it gives the same answers.
An exchange shows two implied probabilities for one selection. Backing at 2.02 implies 49.50% and laying at 2.06 implies 48.54%, and the gap between them is what it costs to cross from one side to the other. Commission on winnings widens that gap, which is why a tool applies its own commission rate before comparing.
Summed across every outcome in one bookmaker's market, implied probabilities come to more than 100%, and the excess is the overround. Putting every price on the same percentage scale is also the plainest way to compare two bookmakers, or a bookmaker against an exchange, on a single selection.
Percentages also make movement readable. A price that shortens from 3.00 to 2.50 has moved from 33.33% to 40.00%, a jump of 6.67 points. The same 0.50 shortening from 10.50 to 10.00 moves the implied chance by only 0.48 of a point, a difference the decimal figures hide.
In the OddsRelay feed
In OddsRelay's feed the inputs are the price fields. On the raw feed a bookmaker's price sits at bookmakers[].markets[].outcomes[].price, and an exchange quotes back_price and lay_price with no commission applied. On the matched feeds they are back[].price and lay[].price, and on the each-way and extra-place feeds the place side has lay prices of its own under lay.place. Read meta.odds_format first, because under oddsFormat=american a price is a signed integer and must go back to decimal before dividing.
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