OGS rating system explained in detail

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Why does OGS use ELO ratings as the main rating system instead of more traditional kyu and dan ranks?

This is a difficult, and slightly misleading question. Most of the kyu / dan systems used on internet go servers have something resembling an ELO rating system behind the ratings seen by the users. The advantage of the ELO system is all the calculations are transparent, and the end result is easily converted to a kyu / dan equivalent, though the accuracy is based largely on the number of players on the server and the spread of those players. It is likely that somewhere in the region of 1000 regular, established players will be required before the ratings are likely to be highly accurate.

Why hasn't my rating changed?

To begin with, you will not have an established rating on this site. This is because without anything to go on it is unlikely an accurate rating could be generated. Ratings are generated when either you have won the equivalent of 1.5 full sized games (smaller boards count less) and lost the equivalent of 1.5 full sized games, or when you have finished the equivalent of 6 full size games. At this point you will have your rating calculated via the method described in the point below. Any games between two provisionally rated players are automatically set as unrated.

It is also important to note that any games that have less than 6 moves played, or have over the handicap threshold (6 stones on a 19x19 board, 4 stones on a 13x13 and 2 stones on a 9x9) will be automatically marked as unrated at game completion. This is to keep the rating system based on the most reliable matches possible and thus to maintain its stability.

How does the rating system work?

Provisional -> Established Ratings

On OGS, to keep it fair for all players, new players start with a provisional rating. To get their first established rating, they must complete a certain number of rated games against non-provisional players that do not end by timeout.

For every full sized game you have won, you are given points equal to their rating, plus 1000 points. This value is divided by 2 for 13 x 13 boards, and divided by 4 for 9 x 9 boards. Similarly, you are awarded points equal to their rating, minus 1000 points for every rated loss. Once the points total has been calculated for all of the games put together, the total is divided by the number of games played (again, smaller boards count proportionally less) to give the first established rating.

Once your rating is established, your rating is adjusted using the ELO formula with K depending on the rating of the winning player in each game. When the game ends in a draw K is determined by the higher rated player - for more details see the description below:

Established Rating System

Once your rating is established on this server, all ratings are adjusted at the end of the game based on the well documented ELO rating system with a variable value for K. The given value of K is calculated separately for each player, and is taken on their rating at the end of the game. The rating used for the winning player then determines K from the following table:

Rating less than 700 -- K Value = 128
700 <= Rating < 1000 -- K Value = 96
1000 <= Rating < 1300 -- K Value = 80
1300 <= Rating < 1600 -- K Value = 64
Rating over 1600 -- K Value = 48

Due to the nature of the smaller K values, the rating fluctuations on individual results are smaller at higher ratings, leading to a compression in rating adjustments the stronger you become.

Ratings between provisional players and provisional / established matches

Due to the impossibilities of calculation, all games played between two provisional players are marked as unrated. Established players do not have their rating adjusted by playing provisional players, unless the provisional player gains an established mid game, in which case the established player will have their rating adjusted, based on the result and the rating of their opponent at the end of the game.

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