A bookmaker’s price is not a clean probability — it is a probability plus margin (the overround). If you want to compare your own read of a match with what the market “really” thinks, you have to take that margin off first. The operation is called devigging (from removing the vig, the commission).
Why “1 over the price” is not enough
Converting a price into a probability with 1 / odds gives numbers that do not add up to 100% — because the overround is baked into them (we covered it separately). For prices of 2.20 / 3.40 / 3.30 the implied probabilities total about 105%. Those surplus 5 points are the operator’s margin, not a real assessment of the chances. For the comparison to be honest, they have to be removed and the total brought back to a hundred.
The proportional method (the simplest one)
The simplest devig is to divide each raw probability by their sum. Taking our example:
raw: 0.4545 / 0.2941 / 0.3030 · total = 1.0516
We divide each by 1.0516:
1: 0.4545 / 1.0516 = 43.2% X: 0.2941 / 1.0516 = 28.0% 2: 0.3030 / 1.0516 = 28.8% total = 100%
And that is it — the total is now exactly 100%, and the numbers can be treated as the probabilities the market actually assigns to the outcomes. Those, not the raw prices, are the honest point of reference.
This is the proportional method: it assumes the margin is spread evenly relative to the probabilities. More refined approaches exist (for instance ones that distribute the margin differently between favourites and outsiders), because in reality the overround often sits more heavily on the less likely results. But the principle is always the same: strip the margin before you start comparing.
What we need devigging for
Devigging is a precondition of an honest model-versus-market comparison. Only once both sides are expressed as “clean” probabilities — ours from the model, the market’s after devigging — can we sensibly ask where they differ, and whether that difference is real or merely noise.
We always do this against the closing line — the final price before kick-off, the best-informed price the market produces. Devigging the closing line gives us the sharpest benchmark available: the best collective assessment of the chances the market managed to reach. That is what we measure ourselves against when we calculate CLV.
And as always, we say it plainly: in historical testing our model does not beat that line. Devigging is not a trick that changes this — it is merely the tool that lets us see the difference honestly, undistorted by margin. If the edge were there, this is exactly how we would see it. For now, it is not.
Educational material. Not advice on taking part in betting. 18+.