The starting ratings are an example. The expected score counts a draw as half a win, so it is not the same as a win probability.
The formulas
- Expected score of the home team: E = 1 ÷ (1 + 10−d/400), where d is the rating difference plus the home advantage.
- New rating = old rating + K × (result − E), with result 1 for a win, 0.5 for a draw and 0 for a defeat.
- K sets how fast ratings react. The World Football Elo Ratings, for example, add 100 points for the home side and use larger K values for more important matches.
How rating systems feed into prediction models is explained in How AI Predicts Football Matches.