At the APSP AGM in 1998 it was agreed to adopt a new rating system, effective from 1 January 1999. The following pages explain how ratings are calculated for an individual tournament and how these are used to calculate the overall rating under the new system. Useful information about all aspects of the ratings system, such as why we have one and how it came about, can be found in this helpful guide, compiled by the ABSP's ratings officer John Grayson.

Ratings Points

Here is an example of a player’s record at a recent tournament:

Round	Opponent	Rating	Won	Lost
1	Player A	158		•
2	Player B	167	•
3	Player C	167		•
4	Player D	167	•
5	Player E	195		•
6	Player F	167	•
7	Player G	160		•
8	Player H	159		•
9	Player I	174	•
10	Player J	156		•
11	Player K	175		•
12	Player L	156	•
Total		2001	5	7

Adding the ratings of all of the player’s opponents gives 2001. The tournament rating is this figure plus 50 points for each game won, less 50 for each game lost; in this example 2001 + 250 - 350 = 1901 tournament rating points. The tournament rating for that event is the average number of rating points per game, which is 1901/12, or 158.42, which rounds to 158.

Note: If players of markedly different ratings play (i.e. more than 40 points apart) then the rating difference is fixed at 40. (For example, if the player, who was rated 169, played someone rated under 129, then he would use 129 as his opponent’s rating for calculating his own rating.)

A further note is required in the instance of playing unrated players. They work out their rating from their first tournament in the manner described above. Their opponents then use that post-tournament rating to work out their own ratings points.

Overall Tournament Rating

The player in our example had a recent history as follows:

Tournament	Games	Rating Points
BMSC	17	2602
Isle of Wight	11	1880
Winter Matchplay	11	2048
Leicester	5	770
East Sussex	5	737
Durham	10	1886
Exeter	19	3003
London	6	1113
Stoke Rochford	12	1901
BMSC	17	2783
Canterbury	6	956

In the Weighted Rolling System, the games are weighted so that the more recent ones count more heavily. The weights used are 150 for the most recent game, down to 51 for the oldest game. Prior to Canterbury, the most recent tournament was the 17-game BMSC, and so these games would be weighted at 150, 149, ..., 134; this is an average weight (AW) of 142. Multiplying the rating points (RP) by the average weight (AW) gives the number of weighted rating points (WRP) for the tournament, which is 395186. The full list of calculations is given in the table below:

Tournament	Games	RP	Weights	AW	WRP
BMSC	4	612	54-51	52.5	32130
Isle of Wight	11	1880	65-55	60.0	112800
Winter Matchplay	11	2048	76-66	71.0	145408
Leicester	5	770	81-77	79.0	60830
East Sussex	5	737	86-82	84.0	61908
Durham	10	1886	96-87	91.5	172569
Exeter	19	3003	115-97	106.0	318318
London	6	1113	121-116	118.5	131890.5
Stoke Rochford	12	1901	133-122	127.5	242377.5
BMSC	17	2783	150-134	142.0	395186
Total	100		10050		1673417

The example player’s rating at this point would be 1,673,417 divided by the sum of the weightings (10,050), to give him an overall rating of 166.51, rounded to 167.

Taking Canterbury into account, the example player drops the 4 games from the 1^st BMSC, and two games from the Isle of Wight, to leave:

Tournament	Games	RP	Weights	AW	WRP
Isle of Wight	9	1538	59-51	55	84590
Winter Matchplay	11	2048	70-60	65	133120
Leicester	5	770	75-71	73	56210
East Sussex	5	737	80-76	78	57486
Durham	10	1886	90-81	85.5	161253
Exeter	19	3003	109-91	100	300300
London	6	1113	115-110	112.5	125212.5
Stoke Rochford	12	1901	127-116	121.5	230971.5
BMSC	17	2783	144-128	136	378488
Canterbury	6	956	150-145	147.5	141010
Total	100		10050		1668641