How it is measured

What is recorded

Bookmakers' own boost pages and the public boost aggregators are read every ten to twenty minutes, around the clock. Each boost is recorded with the time it was first seen, the time it was last seen, the boosted price and, where the bookmaker shows it, the price before the boost. Every response is kept, so any number on this site can be rebuilt from the original pages.

The exchange price

The Betfair exchange is snapshotted every five minutes for every British and Irish race and for football fixtures starting today or tomorrow: match odds, over/under 2.5 goals and both teams to score. For each boost we take the snapshot just before it was first seen, within ten minutes. When a boost appears before the day's first snapshot, typically overnight, it is priced at that first snapshot and labelled "first priced" with the time.

Fair price

For each runner the mid-point of the best back and best lay price gives an implied probability. The market's probabilities are scaled to sum to one, which removes the exchange's own overround. The fair price is one over the runner's scaled probability. For a double or an accumulator the legs' fair prices are multiplied. Exchange commission is not applied: the fair price is a fair price, not a return you could bank on the exchange.

The margin

Margin = boost price ÷ fair price − 1. Positive means the bookmaker offered more than the market's fair price at that moment: genuine value. Negative means the "boost" was still under the market. The same measurement at the boost's last sighting shows how the market moved while it was live.

What cannot be priced

Boosts on markets the exchange does not carry are recorded but not priced: bet builders, player props, cards and corners, correct scores, winning margins, outrights. Boosts on US and Australian racing are excluded because the exchange books on them are too thin to define a fair price. In-play prices are never used; a boost still listed after the off is priced at the off.

Data quality

A margin counts in a published number only when the exchange book behind it held at least £2,000 matched. Thin early-morning books quote a fair price nobody has yet traded, and on them "value" appeared four times as often as on well-traded books when we audited the data; those rows are shown greyed in the tables and left out of every summary.

A margin more than 50% either side of fair is treated as a data problem rather than a finding: a wrong match, or a listing that is not a real price. Such rows are kept but excluded from every number shown. A bookmaker is ranked only once it has 10 priced boosts in the window. Every rule is versioned, and the whole history is recomputed when a rule changes, so numbers for a past day can change and will say so.

Boosts against exchange odds: why the exchange

A bookmaker's own pre-boost price is not a fair reference: it carries the bookmaker's margin and is set by the same desk that decides to boost. The Betfair exchange price is set by thousands of counterparties backing and laying against each other, and on the markets used here (win and place, match odds, over/under 2.5, both teams to score) it is deep enough to define a fair price to within a percent or two most of the day. Comparing a boost with the exchange is the only comparison in which the bookmaker had no hand.

Independence

No affiliate links, no bookmaker advertising, no tips. Prices shown are what was recorded, not what is offered now.