The power method is a de-vig method: it takes the bookmaker's margin out of a market by raising every implied chance to one shared power, k, chosen so the fair chances add up to exactly 100%. Every chance is below 1 and k above 1, so all of them shrink, and a long shot's small chance gives up a bigger share of itself than a favourite's.
How the power method works, in words
- Turn each price into an implied chance, 1 / decimal odds, as how betting odds work explains.
- Add them up. Anything over 100% is margin.
- Raise every chance to a trial power k just above 1 and add them again.
- Raise k if the total is still over 100%, lower it if under, until it is 100%.
- Read each fair price as 1 / fair chance, or as the offered price raised to the power k.
Step 5 works because 1 / (chance raised to k) = (1 / chance) raised to k, so at k = 1.07287 a $9.50 price is fair at 9.50 raised to the power 1.07287 = $11.19. The fair odds calculator finds k for a market of two to 30 prices.
Power method vs scaling on one market
Scaling, which how to remove the bookmaker margin lists first as the multiplicative method, takes the same share off every outcome by dividing each chance by the market total. With an illustrative six-team tournament market at one bookmaker adding up to 110.61%, that share is 1 - 1 / 1.1061 = 9.6%, and k comes out at 1.07287:
| Price | Taken off, scaling | Taken off, power | Fair price, scaling | Fair price, power |
|---|---|---|---|---|
| $2.30 | 9.6% | 5.9% | $2.54 | $2.44 |
| $3.60 | 9.6% | 8.9% | $3.98 | $3.95 |
| $5.00 | 9.6% | 11.1% | $5.53 | $5.62 |
| $9.50 | 9.6% | 15.1% | $10.51 | $11.19 |
| $17.00 | 9.6% | 18.7% | $18.80 | $20.90 |
| $34.00 | 9.6% | 22.7% | $37.61 | $43.96 |
Working for the outsider: scaling gives 34.00 x 1.1061 = $37.61, and the power method 34.00 raised to the power 1.07287 = $43.96.
What the power method does to long shots
The longer the price, the more margin the power method takes off it. Here the $34.00 outsider's fair price is 43.96 / 37.61 - 1 = 16.9% longer than scaling's, while the favourite's is 3.9% shorter (1 - 2.44 / 2.54). The gap can flip a verdict: an offered $40.00 sits 6.4% above the scaled fair price (40.00 / 37.61 - 1) and 9.0% below the power one (40.00 / 43.96 - 1).
A 2017 paper in the American Journal of Sports Science by Stephen Clarke, Stephanie Kovalchik and Martin Ingram tested the power method on three large bookmaker datasets from different sports: it beat scaling on all three and matched or beat the Shin method.
Risk: Betting involves risk. The de-vig method you pick can turn a positive edge into a negative one, and a bet priced above any fair estimate can still lose. See responsible gambling for limits and support.
Where the power method can mislead
- It assumes the margin follows one smooth curve, which a bookmaker shading one popular runner breaks unseen.
- k belongs to one market at one moment: reuse 1.07287 elsewhere, or after a scratching, and the chances stop adding up to 100%.
De-vig settings on B337's screeners
B337's +EV screener builds each sports selection's fair price from prices across the market with the margin stripped out, exchanges and global market references included, and lists the bookmaker prices above it, largest edge first. With the adjustable de-vig you decide which prices feed that fair price and how much margin each gives up. A free account sees the live count and best edge, with the rows locked.
Risk: Betting involves risk. A screener edge is measured against an estimate at the moment the prices were read, prices move before a bet is accepted, and there is no guarantee of profit. See responsible gambling for limits and support.