Probability
A probability measures how likely an event is, from 0% (impossible) to 100% (certain). In football, the probabilities of a match’s three outcomes (home win, draw, away win) add up to 100%. A reliable probability is calibrated: what is rated at 70% happens about 7 times in 10.
In football almost everything is a matter of probability: a home win, a draw, over 2.5 goals, a player scoring. For any match, mutually exclusive outcomes (1, X, 2) must add up to exactly 100%. A probability can also be turned into a « fair price »: 1 / 0.46 ≈ 2.17. Bookmakers’ odds include a margin, so the sum of their implied probabilities goes above 100%.
The quality of a probability is judged by its calibration. Out of 100 outcomes rated at 70%, about 70 should happen; out of 100 rated at 30%, about 30. An outcome at 30% is therefore far from unlikely: it happens almost one time in three. A model whose 30% outcomes never happened would in fact be badly calibrated, not more accurate.
To measure accuracy over time, the Brier score is widely used: for each match, you add up the squared differences between the stated probabilities and the actual result (1 for the outcome that happened, 0 for the others), then take the average. The lower, the better. It rewards both accuracy and honesty: claiming false certainty is costly when you are wrong.
Finally, beware of hindsight bias: after the event, a surprising result always seems explainable. A probability describes the uncertainty before kick-off, not what was « bound » to happen.
Example
A model gives 46% / 26% / 28%, 100% in total. If the home side wins, the Brier score for the match is (0.46 − 1)² + 0.26² + 0.28² ≈ 0.44, against about 0.67 for a prediction giving one third to each outcome. And over 50 matches where an away win is rated at 28%, about 14 away wins are expected.
How Elofoot uses it
Elofoot shows its 1X2 probabilities as whole percentages that always add up to exactly 100%. The dominant outcome is capped at 95%: even with a huge gap in quality, the model never claims certainty. Elofoot's probabilities contain no margin, because Elofoot is not a bookmaker.
Frequently asked questions
- What does a 70% probability mean?
- That across many comparable situations, the event happens about 7 times in 10. So it will fail to happen roughly 3 times in 10: that is not a mistake, it is exactly what the figure says.
- How do you convert a probability into odds?
- Fair decimal odds are the inverse of the probability: 50% gives 2.00 and 25% gives 4.00. Bookmakers’ odds are lower because they include a margin.
- What is the Brier score?
- A measure of the accuracy of probabilistic predictions: the average of the squared differences between stated probabilities and actual results. The lower it is, the better the predictions.
See also
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