OGS - Frequently Asked Questions - OGS Rating System

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1. How does OGS rate players?
2. A variant?
3. How does the rating of a player change the K value?
4. How does that rating certainty change the K value?
5. What does the green (P) after some players' ratings mean?
6. So, how is this rating certainty determined?
7. Which games are counted for ratings?

1. How does OGS rate players?

OGS use a variant of the Elo rating system. 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.

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2. A variant?

The K value in the Elo computation is variable on OGS. It's based on two criteria:

• The rating of the player.
• The rating certainty of players.

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3. How does the rating of a player change the K value?

The K value in the Elo computation is first set to a value depending on your level:

• 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

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4. How does that rating certainty change the K value?

The exact K value used to compute the rating change after a game between player A and player B is:

K' = K * RC(A) / RC(B)

where K is the K value of player A determined given its rating and RC(A) is the rating certainty of player A (same goes with player B).

Rating certainties are between 0.1 and 1 and can be seen in a player profile.

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5. What does the green (P) after some players' ratings mean?

The (P) signifies that the player is provisional (rating certainty below 50%). There are some restrictions placed on provisional players (for example, some games and mini-tournaments can only be joined by non-provisional players).

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6. So, how is this rating certainty determined?

A new player starts with a rating certainty of 0. When you have a rating certainty of 0, your rating is set at the opponent's level +/- 100 depending on whether you win or lose. After one rated game, the rating certainty is set to 0.1.

After a rated game, the rating certainty is incremented by the rating certainty of the opponent multiplied by your rating certainty. If the game ended by time out, the loser's rating drops by 0.1, and the winner's is unchanged.

Each day, the rating certainty is multiplied by an M value (RC = M*oldRC). The current M value is 0.995.

When your rating certainty is under 0.5 (50%), a (P) mark appears next to your name, meaning you are provisionally rated.

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7. Which games are counted for ratings?

If you start a new game by challenging another user directly, or by using the "Start New Game" link, then the game settings allow you to choose whether the game is rated or not. Games that are part of a tournament or ladder competition will automatically count for ratings.

If a game finishes before six moves have been played, then it counts as unrated. Games with "large" handicaps are unrated, where "large" means more than 6 stones on a 19x19 board, more than 4 stones on 13x13 or more than 2 stones on 9x9, and handicap is the number of handicap stones minus floor(komi/6.5). Here, the 'floor(x)' means the greatest integer less than x.

When viewing another user's profile, you can see an estimate of how your rating would change after a game with them. This reflects the change for 19x19 boards. The numbers for 13x13 and 9x9 boards are 1/2 and 1/4 of these values, respectively.

If you wish to get a rating, you may wish to challenge one of the OGS bots: Fuego100k* (12.7 kyu), AyaGo (13.4 kyu) (P), Fuego10k* (17.6 kyu), GnuGo* (19.4 kyu), Fuego1k* (22.8 kyu), GnuGo3* (23.9 kyu), GnuGo2* (27.3 kyu), ELIZABot (30 kyu) (P), LibertyBot* (34.8 kyu), SimpleBot* (38 kyu), WeakBot50k* (45.6 kyu) and BeginnerBot* (46.9 kyu).

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