Odds contain vig, so they cannot be used as probabilities directly. Stripping it is called devigging. The awkward part: there are four mainstream ways to redistribute that margin, and they diverge at long odds — which is precisely where any spread would appear.
A fixture's implied probabilities sum to 104.7%; the extra 4.7 points are the bookmaker's margin. To compare against a prediction market price you must first remove those 4.7 points and recover a set summing to exactly 100%.
| Method | How it allocates | Character |
|---|---|---|
| Multiplicative | scale every outcome proportionally | simplest, most common |
| Additive | subtract the same amount from each | hits longshots harder |
| Power | raise to a power, then normalise | between the two, tunable |
| Shin | assume the book is defending against informed traders and solve that out | strongest theoretical basis |
On the favourite the four barely differ. On the longshot they differ substantially.
Which is the dangerous part, because spreads between a prediction market and bookmaker odds tend to appear on the longshot side. Meaning your choice of devig method decides whether you "find" the opportunity at all.
Put differently: an edge discovered on the longshot side may be nothing more than an artefact of the method you picked — structurally identical to the instrument failure documented in the broken ruler.
There is no universally correct answer, but there is an operable discipline:
| Do | Why |
|---|---|
| Pick one, write it down, do not change it | otherwise you will unconsciously select the one that shows an opportunity |
| Use the other three as a sensitivity check | if the edge disappears under another method, it is not an edge |
| Report the divergence | "all four agree" and "only one works" are completely different strengths of evidence |
The second is the cheapest self-check available: run your finding through all four. Anything that only holds under one is almost certainly a method artefact rather than a market bias.
Basisthis site does not publish which method I use or how I combine sources — that is strategy. The methods themselves are public knowledge, in any textbook.
Before you can devig anything you need odds. The practical situation:
| Issue | Reality |
|---|---|
| Coverage | good for major leagues, poor for niche sports |
| Esports | mainstream odds APIs have zero coverage |
| Update frequency | tiered by subscription; faster costs more |
| Historical data | usually charged separately, and frequently has gaps |
| Free sources | exist, but laggy, narrow, and liable to break |
Devig algorithms are public knowledge and a few lines of code. Obtaining timely, clean, sufficiently broad odds is the hard part — and it costs money.
So when evaluating this path, compute the data cost first: if a month of odds subscription exceeds your expected monthly return, there is no point continuing. That judgement can be made before writing a line of code.
Turning a candidate into an opportunity requires three gates:
| Gate | Question |
|---|---|
| Cost | does the gap cover the taker fee plus the spread? |
| Executable | is there size, or only a quote with no depth? |
| Robust | does it survive another devig method? another odds source? |
The third is this entry's point: anything that disappears under a reasonable change of basis was never an edge.
Devigging is mandatory, but "how" has four defensible answers that diverge most at long odds — exactly where spreads live. So run any longshot-side finding through all four; one that holds under only one is a method artefact, not a market bias.