# chess online

GameKnot related: Rating change and draws
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pianoenzo
05-Oct-10, 21:57

Rating change and draws
I am trying to find out how the rating change works when there is a draw. Can anyone explain it to me, as I do not find it in frequently asked questions...
 easy1905-Oct-10, 23:14

there is this. :)
GameKnot rating system is based on Elo rating system with a fixed K = 20 and the following modifications:

The first 20 games are used to establish player's rating on the website. During the first 20 games, the player's rating is calculated as an average of the ratings of all his/her opponents, +400 in case of a win, -400 in case of a loss, same for a draw. +/-200 points are used when playing against a player with a provisional rating.
PLayer's rating is provisional during the first 20 games, after which it becomes established.
Player's rating is considered to be equal to 1200 during the first 5 rated games.

Timeouts are counted as wins only if there were at least 3 moves made in the game (losses are always counted for the timed out players, regardless of how many moves were made).

The higher of two ratings, at the beginning of the game and at the end, is used to calculate the rating adjustments after the game is over.

For example, if during your first 20 rated games, you played 3 games and you won against 1200 player with provisional rating, then against 1400 player with established rating, but lost against 1600 player with established rating, your rating will be:
( (1200 + 200) + (1400 + 400) + (1600 - 400) ) / 3 = 1467

Or, if during your first 20 rated games, you win against 1200 provisional, win against 1400 established, lose against 1600 provisional, draw against 1500, your rating will be:
( (1200 + 200) + (1400 + 400) + (1600 - 200) + 1500 ) / 4 = 1525
algol
06-Oct-10, 06:30

Deleted by algol on 06-Oct-10, 06:31.
 algol06-Oct-10, 06:34

...
Most of the text in the FAQ is about the provisional rating. Since you have more than 20 games, the Elo rating system is used which is based on this formula:

Expected score for the strongest player = 1 / (1 + power10((rating weakest - rating strongest)/400))

For example, your rating was 1235 in a game you played against thebishop1000 who was then rated 1271. The expected score for the strongest player is 1 / (1 + power10((1235-1271)/400) = 0.55

The rating gain for the strongest player is K * (actual score - expected score).

K = 20 for the gameknot rating calculations as easy19 points out. Because the game was drawn, the actual score is 0.5. We get a rating change for thebishop1000 equal to 20 * (0.5 - 0.55) = -1.0 rating point.
For the other player the expected score is (1 - 0.55) = 0.45 and a draw gives a rating change 20 * (0.5 - 0.45) = +1.0 rating point.

A win will always give you 10 rating points more than a draw would, and a loss always 10 rating points less than a draw.