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1. Ratings system concepts

The chief design goal of this rating system is to provide an objective measure of the relative DBM playing strength in a competitive environment of each rated player. The ratings should be as fair as possible to all players regardless of their ability to attend competitions or level of playing success.

This system of calculating Australian DBM ratings is based on the Glicko system used in internet chess which is itself a statistically sophisticated refinement of the Elo system used in international chess.

Under this system two values are calculated for each player.

The most important one is the rating level of the player. This is a numeric value representing the estimated relative playing strength of each player. The higher the value, the stronger the player. If two players are equally rated, then based on the information to date, there is expected to be a 50% chance of either player ultimately defeating the other in a competitive game of DBM. The greater the difference in rating levels, the greater the expectation that the higher rated player will win a competitive game of DBM. In statistical terms, the rating level is a central or unbiased estimate of each player’s DBM playing strength, as likely to be an overestimate as an underestimate.

The second value calculated is a reliability measure. This is an indicator of how accurate an estimate the rating level is likely to be, or in other words, how much trust can be placed in it. It is similar to a standard deviation. As a player plays more games in rated competitions the ratings level becomes more reliable however the reliability measure is subject to decay with time. That is, the longer a player goes without competing in a rated competition the less reliable his estimated rating level becomes.

In the reported ratings, all rating levels have been rounded to the nearest multiple of 25 (to avoid giving an impression of spurious accuracy) and the reliability of each rating has been categorised as one of A, B, C, D or E. Those rating with reliability of A are the most reliable and stable while those with a reliability of E are little more than early indicators and could be subject to large fluctuations as more results are received. Typically a player who has played 20 rated games over the last 2 years would be in category C and one who has played more than 30 would be in category B.

Each player’s rating level is calculated based on results from each valid game played in rated public competitions. The previous rating level is adjusted depending upon whether the player has performed better or worse than expected based on their past results and the quality of the opponents they faced in each competition.

A player does not have to win every game to improve his rating. A mix of wins and losses may improve the rating of an average or weaker player. Similarly a stronger player does not necessarily have to win every game to maintain their high rating although they would normally be expected to win a higher proportion than a weaker player.

A player with a long established track record will have a stable rating level. One bad tournament will not cause that rating to plunge drastically. Similarly one good tournament will not cause it to soar to exceptional heights.

Byes and forfeits are ignored. The DBM 10 point scoring system is always used for rating purposes although competition organisers are free to use whatever system they like to determine their placings and prizes. The calculations neither use competition placings nor are they expected to be used in determining competition placings. The rating system should not interfere with or change the way any competition is run.

2. Rated competitions

Only results from rated competitions are used for rating purposes.

Rated competitions are those that are open to the public, have at least 7 competitors for 3 rounds, are genuinely competitive with some form of prizes or prestige at stake and have a minimum proportion of previously rated players (waived for the initial rating calculations obviously). Both players must have the same AP in each game, although AP levels can vary between games. There must be no special rules that unfairly disadvantage particular players (not armies) and other special rules should be publicised beforehand.

The competition organiser has to agree to make sufficient results available in some format to enable their inclusion in the rating calculations.

At the moment, it is proposed to give Cancon slightly more prominence than other Australian competitions. For players with very reliable ratings (generally category B) Cancon results will have slightly more weight than usual. Also players will not be allowed to opt out of Cancon for rating purposes (see below). For Cancon to be given this special treatment it has to continue to be over 6 rounds, include at least 60 players overall and at least 5 players from each of 4 different states and territories. If another competition meets this criteria, then consideration will given to giving it similar special treatment.

3. Published rating information

The published information will consist of 2 lists. The most important one is an alphabetical list of all rated players with their current rating information. The second is a list of the top rated players that have a minimum reliability of category C, in order of rating level. This means that players who perform magnificently in their first rated competition will not be included unless they also perform well in further competitions.

Revised ratings will be published as soon as possible after results are received for each rated competition. The published information will include the new ratings, movements and actual and expected scores from the latest competition.

Only players who have taken part in a rated competition in the last 25 months will have their rating information published. The information on such players will be retained should they resume playing.

Any player who does not wish to have their DBM rating information published may write to me at the e-mail address below. Their ratings will not be published however it will still be calculated and used in calculating other player’s rating information.

4. Opting out

Players may elect to opt out of ratings for a particular rated competition other than Cancon. The usual reason for doing so would be to allow them to take a "fun" or "patheticon" army without endangering their ratings.

To opt out, a player must contact me (see e-mail address below) prior to the commencement of the competition. I will acknowledge this as soon as possible. It is the player’s responsibility that an election to opt out is successfully sent and received.

Players may only opt out of 2 tournaments in any 18 month period.

5. New players

New players are rated after their first rated competition. In Australia, new players are given a initial rating of 2,000 and a reliability measure that indicates that this has virtually no credence. At the end of their first rated competition, their rating level will be published based on their results in that competition and will normally still be in reliability category E (most unreliable).

The early ratings of new players are very unreliable and thus can be subject to big fluctuations over their first few competitions. For this reason new players should not be at all discouraged if their first published ratings are quite low if the player takes a little while to adjust to tournament play. Some reasonable results later should soon see their ratings return to more acceptable levels.

6. Further information

Anybody who wants further information on this topic or has comments or suggestions, should feel free to contact me by e-mail at david@yass.fam.aust.com. Information on this and other specifically Australian DBM topics may also be available through the Oz DBM mailing list (OzDBM@listbot.com) which can be joined through Doug "Maddog" Melville’s Australian DBM web-site at http://dbm.freehosting.net. I hope that Doug will also publish the ratings and any other related information on his web-site when he has an opportunity.