I’ve heard it’s a standardized scoring method for many more competitions than just chess. But what is it and how does it work?
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From a practical standpoint, before a game every competitor has a probability of winning, calculated using the competitor’s rating. After the game, the system compares the probability calculated pre-game with the actual result, and modifies the rating depending on the result (adding points if they won, subtracting them if they lost, and generally they’re the same for both competitors), in such a way that a probable result will exchange few points and an unprobable one will exchange a lot. This will give the new rating, that’ll be used for the next game.
There are several Elo-based systems, which differ on how many points to exchange per game, how many points are “one order of magnitude” of probability (i.e. when a competitors has a 10 in 11 probability of winning, and the other a 1 in 11), if there are games with different importance, etc. But the idea is the same: there’s a game, a probability is calculated before the game, and after the game they calculate the exchange of rating points.
The Elo system’s spirit is that there’s a “true rating”, which cannot be known, and which will tell you who will win a game. Now, we only have access to the score of a game. Using those scores as input, Elo tries to get closer and closer to the “true rating”. Of course, this “true rating” is always moving, so you never reach it.
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