It's easy to imagine a bookmaker sitting in a back room deciding what each horse "should" be. The reality is closer to the opposite: a price is mostly a crowd's estimate of a horse's chance, nudged into shape by money. Understanding how that estimate forms is the first step to spotting when it's wrong.
The opening price
Bookmakers and exchanges open a market with an initial guess — informed by ratings, form and their own models. It doesn't need to be perfect, because the next stage does the heavy lifting.
Money moves the price
As bets arrive, prices shift: heavily backed horses shorten, neglected ones drift. This is the market aggregating thousands of opinions — recreational punters, syndicates, models, stable confidence — into a single number. The more money and the more informed it is, the sharper that number becomes. (See steamers and drifters for how to read those moves.)
The overround
Lay out a bookmaker's prices and the implied probabilities sum to more than 100%. That excess — the overround or "margin" — is their built-in edge. It's why simply being right slightly more than half the time isn't enough to profit: you have to beat the price and the margin.
Why the closing line is so sharp
By the off, the market has absorbed nearly all available information. The closing price is the crowd's best, final estimate — and it's very good. That's exactly why beating it consistently is the real test of skill, and why we measure ourselves with closing-line value rather than a run of winners.
Where the opportunity is
If the closing line is efficient, value lives before it forms — in early prices, in structural quirks like the favourite–longshot bias, and in markets that attract less sharp money. A model's job is to get there first with an honest probability. That's the whole game. See how our model works.