At the 1998 AGM it was agreed to adopt a new rating system, effective from January 1 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.

From the introduction of the new system until December 31st 2010, the calculation was based on each player's 100 most recent games, with weightings of 150 to 51.

From January 1st 2011, the calculation uses 150 games where available, with weightings of 225 to 76.

An example at the end of this file shows a calculation based on fewer than 150 games.

Ratings Points

Here is an example of Player X's record at the Penzance tournament:

Round	Opponent	Rating	Won	Lost
1	Player A	161	•
2	Player B	157	•
3	Player C	154	•
4	Player D	167	•
5	Player E	155		•
6	Player F	167	•
7	Player G	160		•
8	Player H	159		•
9	Player I	154	•
10	Player J	156		•
11	Player K	155		•
12	Player L	156	•
Total		1901	7	5

Adding the ratings of all of the player's opponents gives 1901. The tournament rating is this figure plus 50 points for each game won, less 50 for each game lost; in this example 1901 + 350 - 250 = 2001 tournament rating points. The tournament rating for the event is the average number of rating points per game, which is 2001/12 = 167 rounded.

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 she would use 129 as her opponent's rating for calculating her 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.

Finally, the effect of the lowest allowed rating, currently 50, is that the lowest allowed tournament performance rating is also 50. In other words, if the calculation of a performance is below 50, a value of 50 is used instead.

Overall Tournament Rating

Before Penzance, Player X's games history was as follows (most recent first):

Tournament	Games	Rating Points
BMSC	22	3333
Naseby	27	4062
Thurso	7	1178
Polperro	22	3156
Lowestoft	6	880
Nomads	16	2427
Eskdalemuir	21	3219
Lerwick	7	1016
Fishguard	22	3416

In the Weighted Rolling System, the games are weighted so that the more recent ones count more heavily. The weights used are 225 for the most recent game, down to 76 for the oldest game where the full quota of 150 games is available. The most recent tournament is the 22-game BMSC, and so these games would be weighted at 225, 224, ..., 204; this is an average weight (AW) of 214.5. Multiplying the rating points (RP) by the average weight (AW) gives the number of weighted rating points (WRP) for the tournament, which is 714928. The full list of calculations is given in the table below:

Tournament	Games	RP	TrnPrf	Weights	AW	WRP
BMSC	22	3333	152	225-204	214.5	714928
Naseby	27	4062	150	203-177	190	771780
Thurso	7	1178	168	176-170	173	203794
Polperro	22	3156	143	169-148	158.5	500226
Lowestoft	6	880	147	147-142	144.5	127160
Nomads	16	2427	152	141-126	133.5	324004
Eskdalemuir	21	3219	153	125-105	115	370185
Lerwick	7	1016	145	104-98	101	102616
Fishguard	22	3416	155	97-76	86.5	295484

Player X's rating at this point would be 3,410,177 (sum of WRP) divided by the sum of the weightings (22,575), to give a rating of 151 rounded.

For the new rating after Penzance, 12 games from the oldest tournament, Fishguard, are discarded. The new calculation is:

Tournament	Games	RP	TrnPrf	Weights	AW	WRP
Penzance	12	2001	167	225-214	219.5	439220
BMSC	22	3333	152	213-192	202.5	674932
Naseby	27	4062	150	191-165	178	723036
Thurso	7	1178	168	164-158	161	189658
Polperro	22	3156	143	157-136	146.5	462354
Lowestoft	6	880	147	135-130	132.5	116600
Nomads	16	2427	152	129-114	121.5	294880
Eskdalemuir	21	3219	153	113-93	103	331557
Lerwick	7	1016	145	92-86	89	90424
Fishguard	10	1552	155	85-76	80.5	124995

Note - because 12 of Fishguard's games have been discarded, the 1552 in the RP column is calculated as (3416 * 10 / 22). Player X's rating would now be 3,447,656 divided by 22,575, which is 153 rounded.

Finally, an example of the weightings used when a player has fewer than 150 games.

Tournament	Games	RP	TrnPrf	Weights	AW	WRP
Liverpool	11	1468	133	225-215	220	322960
Nailsea	7	1016	145	214-208	211	214376
Lincoln	7	1111	159	207-201	204	226644

The same calculation (sum of the WRP column divided by the sum of weights) is used as for the previous examples.
The values here are 763980 divided by 5325 which is 143 rounded.