The Uppsala Mahjong rating system is a system used to calculate an estimate of the strength of the player, based on his or her performance versus other players. A higher number indicates a stronger player. Players' ratings go up if they perform better than expected and down if they perform worse than expected. The magnitude of the change depends on the rating of their opponents..
Every new player starts with a rating of 50.
At every game you do pair-wise comparisons between all players. If you win against a player of the same level you get 1 point. If you win against a player with higher rating you get a little more than 1 point, and if you win against a player with lower rating you get a little less than 1 point. If you lose against a player of the same level you lose 1 point. If you lose against a player with lower rating you lose a littler more than 1 point, and if you lose against a player with higher rating you lose a little less than 1 point.
This computes the score if all mahjongs were worth exactly 8 points. If you win a hand you get +32 points, if you give away the mahjong tile you get -16 points, and for all other you get -8 points (including self-drawn, but not draws).