The Glicko rating system and Glicko-2 rating system are methods for assessing a player’s strength in games of skill, such as chess and Go. It was invented by statistician Mark Glickman 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.
Both Glicko and Glicko-2 rating systems are under public domain and found implemented on game servers online (like Pokémon Showdown, Chess.com, Counter Strike: Global Offensive, Team Fortress 2, and competitive programming competitions. The formulas used for the systems can be found on the Glicko website.
The RD measures the accuracy of a player’s rating, with one RD being equal to one standard deviation. For example, a player with a rating of 1500 and an RD of 50 has a real strength between 1400 and 1600 (two standard deviations from 1500) with 95% confidence. Twice (exact: 1.96) the RD is added and subtracted from their rating to calculate this range.
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 Daily Omnivore 
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