   Next: Standard rating formula Up: Details of the Rating Previous: Effective number of games

## Special rating formula

This procedure is to be used for players with either , or players who have had either all wins or all losses in all previous rated games.

The algorithm described here extends the old provisional rating formula by ensuring that a rating does not decrease from wins or increase from losses. In effect, the algorithm finds the rating at which the attained score for the player equals the sum of expected scores, with expected scores following the provisional winning expectancy'' formula below. For most situations, the resulting rating will be identical to the old provisional rating formula. Instances that will result in different ratings are when certain opponents have ratings that are far from the player's initial rating. The computation to determine the special'' rating is iterative, and is implemented via a linear programming algorithm.

Define the provisional winning expectancy,'' , between a player rated and his/her -th opponent rated to be Let be the prior'' rating of a player (either the pre-event rating for rated players, or the Step 1 imputed rating for unrated players), and be the effective number of games. Also let be the number of games in the current event, and let be the total score out of the games (counting each win as 1, each loss as 0, and each draw as 0.5).

The variables and , which are the adjusted initial rating and the adjusted score, respectively, are used in the special rating procedure. If a player has competed previously, and all the player's games were wins, then let If a player has competed previously, and all the player's games were losses, then let Otherwise, let The objective function which is the difference between the sum of provisional winning expectancies and the actual attained score when a player is rated , is equal to 0 at the appropriate rating. The goal, then, is to determine the value of such that within reasonable tolerance. The procedure to find is iterative, and is described as follows.

Let be a tolerance to detect values different from zero. Also, let , , and, for , , . Denote the unique and , , as the collection If there are no duplicates, then . These values are the knots'' of the function (essentially the value where the function bends'' abruptly).

1. Calculate This is the first estimate of the special rating (in the actual implemented rating program, is set to , but the final result will be the same - the current description results in a slightly more efficient algorithm).
2. If , then
1. Let be the largest value in for which .
2. If , then set . Otherwise, calculate • If , then set , and go back to 2.
• If , then set , and go back to 2.
3. If , then
1. Let be the smallest value in for which .
2. If , then set . Otherwise, calculate • If , then set , and go back to 3.
• If , then set , and go back to 3.
4. If , then let be the number of , for which Additionally, if , set .
1. If , then exit.
2. If , then let be the largest value in and be the smallest value in for which . If
• , then set .
• , then set .
• , then set .
If the final value of is greater than 2700, the value is changed to 2700. The resulting value of is the rating produced by the special'' rating algorithm. Note that these computations do not depend on whether the event is a full-K'' or half-K'' event.   Next: Standard rating formula Up: Details of the Rating Previous: Effective number of games
Mark Glickman
2004-09-22