Qualifying loss
Qualifying loss is the small amount a matched back-and-lay bet costs whichever way the result goes, usually paid to earn a bookmaker free bet.
Two things create it. The lay price is usually a little above the back price, and the exchange takes commission from the lay bet when it wins. Sized correctly, the lay makes the loss the same whether the selection wins or loses, so the result stops mattering and only the cost remains.
The lay stake is back stake × back odds ÷ (lay odds − commission). Back 10 units at 3.0 and lay at 3.1 at 2% commission (use your own rate), and the lay stake is 10 × 3.0 ÷ 3.08 = 9.74 units.
If the selection wins, the back bet makes 20 units and the lay costs 9.74 × 2.1 = 20.45. If it loses, the lay wins 9.74 less commission, 9.55, and the 10-unit back stake is gone. Either way the loss is 0.45 units, or 4.5% of the stake, and a tool ranking qualifiers sorts on that share.
In the OddsRelay feed
In the feed, both prices sit on one outcome of a matched feed. back[] lists bookmaker prices best first and lay lists exchange prices lowest first, so back[0].price and lay[0].price give the smallest gap on offer for that selection. The commission is the customer's own figure, and the feed carries no loss or rating field, since both depend on it.
The lowest lay price is not always the cheapest lay. A user paying different commission at different exchanges works out each entry in lay at its own rate, keyed on lay[].exchange. Size matters too. lay[].available is the money available at that price, and a lay stake bigger than it either waits unmatched or goes at a worse price, which raises the loss.
The figure is only as current as its prices. meta.processed_at gives the response's time and meta.last_seen each bookmaker's last read, so a tool can show how old a pair is and prompt a check on the bookmaker's page before the back bet goes on.
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