How does the ELO system work?

A player’s ELO rating is represented by a number which increases or decreases depending on the outcome of games between rated players. After every game, the winning player takes points from the losing player. The difference between the ratings of the winner and loser determines the total number of points gained or lost after a game. In a series of games between a high-rated player and a low-rated player, the high-rated player is expected to score more wins. If the high-rated player wins, then only a few rating points will be taken from the low-rated player. However, if the lower rated player scores an upset win, many rating points will be transferred (the larger the gap between the player’s ratings, the more the resulting player rating will fluctuate). This means that this rating system is self-correcting. A player whose rating is too low should, in the long run, do better than the rating system predicts, and thus gain rating points until the rating reflects their true playing strength.

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