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The Glicko rating system and the Glicko-2 rating system are similar to the Elo rating system: a method for assessing a players strength in games of skill such as chess. It was invented by Professor Mark E. Glickman as an improvement of the Elo rating system. The main idea is the introduction of a measurement for the 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. The formulas used for the systems can be found on the Glicko website.

The RD measures the accuracy of a player's rating. For example, a player with a rating of 1500 and an RD of 50 will have a real strength between 1400 and 1600 with a chance of 95%. Thus twice the RD is added and subtracted from his rating to calculate this range. Also the rating change after a game depends on the RD. It will be smaller if the player's RD is low and his opponent's RD is high. This is because in the first case, the player's rating is already known well, while in the second case the opponent's rating is not very exact and does not give much information about the first player's strength. The RD 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.

• Professor Glickman's Glicko-Website
• Research Papers by Professor Glickman including one on the Glicko system.