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: Global Offensive, Team Fortress 2, Dota 2, Guild Wars 2, Splatoon 2, Online-go.com, Lichess and Chess.com.
The Reliability 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 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 Glicko-2 rating system improves upon the Glicko rating system and further introduces the rating volatility σ. A very slightly modified version of the Glicko-2 rating system is implemented by the Australian Chess Federation.