The rating system

A rating is an approximate measure of a player's strength. A higher rating means a better player. On ISC the average rating is around 900, expert players are around 1800.

The formula used for calculating rating changes depends if you have an established rating or a provisional one.A player is provisional if he doesn't have 11 rated games. To check how many rated games you have type on command finger. Only the games played against established players are rated. After completing your first game against an established player you will get a provisional rating.

provisional vs provisional
If both players have a provisional rating the game will not be rated, and it will not count in stats.

provisional vs established
The formula used for the provisional player is based on the average rating of his opponents and the number of his own wins and losses. This formula can produce strange results since you can lose points after winning a game if your opponent’s rating is below average.
For a provisional player the BEST column in the finger info shows the average rating of the opponents.
If this average is 1231 and you won 4 games and lost 4, your rating will be 1231. If you won 3 games and lost 0, your rating will be 1629.
If you’re the established player, the rating points that you would normaly win it will be divided by (11 - n), where n is one plus the number of games your opponent has played.

established vs established
The rating formula used when both players have an established rating is based on the opponent’s rating and the score difference. You can win or lose anywhere between 0 and 40 points.
If your opponent has the same rating as you and you win with a score difference of 1 point, you will get 11 rating points. For a score difference of 200 points you will get 21 rating points.

You can see how your rating will be affected before the game starts. Click on the person’s name and select Assess. Alternatively, you can type assess name in the command mode.

The assess command will show the rating changes for every dictionary.

rating - to see the formula used for calculating rating changes