Club Ratings — Explanation

Club ratings are not related to an individuals NZCF or FIDE rating. Ratings are calculated using the players prior rating and the rating of players beaten or lost to after each rated event. Only financial members receive a rating.  All club tournaments, including weekend events, are included for ratings purposes — so participating in the rookies tournaments may boost members ratings substantially. Members who are in arrears for their annual subscription will continue to be rated but updated ratings will not be published on the website.

Ratings are based on the ELO system as follows

If Player A has true strength R_A and Player B has true strength R_B, the ELO rating system assumes the expected score of Player A is

E_A = \frac 1 {1 + 10^{(R_B - R_A)/400}}.

Similarly the expected score for Player B is

E_B = \frac 1 {1 + 10^{(R_A - R_B)/400}}.

Also note that

E_A + E_B = 1.

In practice, since the true strength of each player is unknown, the expected scores are calculated using the player’s current ratings. When a player’s actual tournament scores exceed his expected scores, the Elo system takes this as evidence that player’s rating is too low, and needs to be adjusted upward. Similarly when a player’s actual tournament scores fall short of his expected scores, that player’s rating is adjusted downward.  With club ratings the maximum possible adjustment per game, called the K-factor, is initially set at 32 points. When a player has played more than 30 rated games the K-factor reduces to 16.

Supposing Player A was expected to score E_A points but actually scored S_A points (which can be 1, 0.5, or 0 for an individual game)  the formula for updating his rating is

R_A^\prime = R_A + K(S_A - E_A).

All new players are assigned a rating of 1500 points. Any guests participating in an event are arbitrarily assigned a rating of 1500 points for rating purposes (so playing them and winning can be an easy way to pick up ratings points!!!). The maximum amount a rating can change in one game is 32 points. There is a ratings floor: no rating is allowed to drop below 1000. If two opponents with the same rating draw their game, neither rating changes. The points gained by one opponent equals those lost by the other (in a single game) provided their K-factors are equal.


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