- 28 Aug '03 09:20You're right - it's the provisional rating system. For your first 20 games, your rating is the average of your opponents' ratings for those games, plus or minus 400 depending on whether you won/lost.

So say you play 4 games against opponents rated 1200 (you win), 1000 (you lose ), 1400 (you win) and 1800 (you draw), then your provisional rating will be the average of:

1200 + 400 = 1600

1000 - 400 = 600

1400 + 400 = 1800

1800 + 0 = 1800

which is 1450.

Then let's say you have a rematch with the 1000-rated player (must avenge that embarrassing defeat!) and beat him :}

Then your rating averages the other results AND this one (at 1400). So your rating will go down to 1440

Once you reach 20 games another formula gets used, so this shouldn't happen again.

Hope that all makes sense, think I've made a mountain out of a molehill there!

Jim - 28 Aug '03 10:22I think the wins will all have fixed values - if he was 1200 when you first beat him, you should get 1600 for that win even if his rating changes.

However, next time you beat him, you got scored at his new (lower) rating +400, which must have still been below your current rating.

(I recall reading somewhere that you only get +/- 200 points when playing players with provisional ratings - not 100% sure on that but that might explain it?)

Jim - 28 Aug '03 11:39 / 1 editHere is the help file entry which explains the new rating system.

When you join, you are given a rating of 1200.

If you enter into a rated game, your rating will be recalculated on its completion so long as at least two moves have been made by each player.

Until you complete 20 games, you will have a provisional rating. This rating is calculated as the average of all the games you have played. Each game is scored as being your opponent's rating (for a draw), your opponent's rating plus 400 (for a win), or your opponent's rating minus 400 (for a loss). If your opponent is also on a provisional rating, then the 400s above become 200s. If your opponent has played fewer than five games then their rating is treated as 1200 when calculating your rating.

If you have a non-provisional rating and you play a provisional-rated player, then you receive (or lose) only half the number of rating points you would normally. If the provisionally-rated player has played fewer than five games, then their rating is treated as 1200 when calculating your rating.

Players are rated using the following formula:

New Rating = Old Rating + K * (Score - Win Expectancy)

K is a constant (32 for 0-2099, 24 for 2100-2399, 16 for 2400 and above)

Score is 1 for a win, 0.5 for a draw and 0 for a loss.

The Win Expectancy is calculated using the following formula:

Win Expectancy = 1 / (10^((OpponentRating - YourRating)/400)+1)

The Win Expectancy is used in the rating calculation but is interesting on its own. For example, the calculation below is for a rating difference of 200. This shows that if you have a rating 200 points higher than another player, you can expect to beat them, on average, three times for each four games played.

Win Expectancy = 1 / (10^(-200/400)+1) = 0.76

Note: ^ = "to the power of", e.g. 2^3=8.