Every bookmaker builds a margin (also called overround or vig) into their odds. This margin is how they guarantee profit regardless of the outcome. Shin devigging is the mathematical method to remove this margin and reveal the true implied probabilities.
The Problem
Consider a football match where the true probabilities are Home 50%, Draw 25%, Away 25%. Fair odds would be 2.00, 4.00, 4.00. But a bookmaker with 5% margin might offer 1.90, 3.60, 3.60. The sum of implied probabilities is 105% — the extra 5% is the bookmaker's edge.
Why Simple Methods Fail
The naive approach is to divide each probability proportionally to remove the margin. But this assumes the margin is distributed equally across all outcomes — it is not. Bookmakers load more margin on the favourite (because that is where most money flows) and less on outsiders.
The Shin Method
James Shin developed an iterative method that accounts for this non-linear margin distribution. The algorithm:
- Start with implied probabilities from raw odds
- Estimate the overround parameter (z)
- Adjust probabilities based on z
- Recalculate z from adjusted probabilities
- Repeat until convergence (typically 10-20 iterations)
The result is a set of fair probabilities that sum to exactly 1.0, with the margin removed in a mathematically rigorous way.
Why This Matters for Value Betting
Once you have true probabilities, you can calculate fair odds (1/probability). If any bookmaker offers odds higher than fair, you have a value bet. At Athenea Apex, we run Shin devigging on every market across 47 leagues in 6 sports, every 5 minutes.