Expected value (EV)

For a back bet the arithmetic is short. EV = stake × (chance × price − 1), with the chance as a decimal. A positive figure means the price is above the fair price for that chance, which is how value betting is defined, and a negative one means it is below.

Take a selection a tool puts at 50%, a fair price of 2.00. At 2.20, a 10-unit bet has an EV of 10 × (0.5 × 2.2 − 1) = 1.00 unit. At 1.90 the same bet's EV is 10 × (0.5 × 1.9 − 1) = −0.50. Neither figure is what one bet does. At 2.20, a single bet ends 12 units up or 10 down.

The chance is the hard part. EV is only as sound as the probability behind it, which comes from the tool's own model or from a reference market with its margin removed, and a small error in the chance moves the answer a long way. At 2.20, a chance of 45% instead of 50% turns +1.00 into −0.10.

In the OddsRelay feed

OddsRelay's feed carries prices and no EV field, because the chance is the tool's own. The inputs are the price fields: back[].price on the matched feeds, a bookmaker's outcomes[].price on GET /v2/odds/raw, and each exchange's back_price and lay_price on raw, which a tool can use as its reference once it allows for commission and the gap between back and lay. meta.odds_format says whether a reply is decimal or American, and an American price goes back to decimal before the formula.

Each figure holds only while its price does. On the matched feeds meta.last_seen gives each bookmaker's last read per sport, and on raw last_update says when a bookmaker's market last changed, so a tool can show the age of the price beside its EV.

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