Glicko rating system
Rating system for players of skill-based games
The Glicko rating system and Glicko-2 rating system are methods of assessing a player's strength in zero-sum two-player games. The Glicko rating system was invented by Mark Glickman in 1995 as an improvement on the Elo rating system and initially intended for the primary use as a chess rating system. Glickman's principal contribution to measurement is "ratings reliability", called RD, for ratings deviation.
Overview
Mark Glickman created the Glicko rating system in 1995 as an improvement on the Elo rating system.
Both the Glicko and Glicko-2 rating systems are under public domain and have been implemented on game servers online like Counter-Strike 2, Team Fortress 2, Dota 2, Guild Wars 2, Splatoon 2, Pokémon Showdown, Online-go.com, Free Internet Chess Server, Lichess, Chess.com, and TETR.IO.
The Ratings Deviation (RD) measures the accuracy of a player's rating, where the RD is equal to one standard deviation.
For example, a player with a rating of 1500 and an RD of 50 is expected to have a real strength between 1402 and 1598. To calculate this range, the RD is added and subtracted 1.96 times from their rating to arrive at the 95% confidence interval. After a game, the amount the rating changes depends on the RD: the change is smaller when the player's RD is low (since their rating is already considered accurate), and also when their opponent's RD is high (since the opponent's true rating is not well known, so little information is being gained).
The RD itself decreases after playing a game, but it will increase slowly over time of inactivity.
The Glicko-2 rating system improves upon the Glicko rating system and further introduces the rating volatility σ.
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