ASPCC Ratings
Kristo S. Miettinen
Ratings Report for KK288 -- Ratings Order |
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All players with established ratings (10+ games against opponents with
established ratings) and current membership. An initial rating was
assigned to Charles Carter (congratulations!). |
All players with provisional ratings (0-9 games against opponents with Greatest increases (>20 points). *These
increases (and other enormous increases in previous reports) are
due to the assignment of an initial rating to a previously provisionally
rated player. When an initial rating is assigned, the provisional
rating is used only as the starting point of an algorithm that searches
for a rating consistent with the batch of established results. In
this consistency check, the original provisional rating is assigned
no weight, and therefore initial ratings can depart from provisional
ratings by arbitrarily large amounts, if a player’s results warrant
such a departure.
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Overview of the rating systemThe rating system implemented for ASPCC is Arpad Elo’s system with a fixed weight per game and logistic scoring expectation function. For games played between players with established ratings, this means essentially that the game is a wager for 32 rating points, with each player contributing a portion of the 32 points to the “pot” in proportion to their odds of winning (estimated from previous ratings). The 32 points then go to the winner (minus the winner’s contribution to the pot), while the loser loses their contribution to the pot. In the event of a draw, each player gets 16 points, minus their contribution to the pot. The contributions are taken out at the same time as the game is rated, i.e. there is no “pay now, win later” delay in adjusting ratings. The scoring expectation function and the contribution to the 32-point pot can be computed for each player form the following formula: Expected score =
1/(1+exp[(his_rating-my_rating) /166.2]) The expression “exp” represents the exponentiation function, available in Excel with that name, or on any scientific calculator with the key labeled “ex”. To take an example, suppose that player A has a rating of 1450, and
player B has a rating of 1320, and they play a game. What happens in
each of the three cases (win/lose/draw)? So, to make the example concrete, let’s stipulate that the ratings for player A and player B in the example were published in KK265, and so was the result of their game, and neither player A nor player B had any other results published in KK265. Then, we can calculate the expected result of their game: player A’s expected score is 0.686, while player B’s expected score is 0.314. Each player contributes rating points to the wager in these proportions, so player A contributes 22 points, while player B contributes 10 points (for a total of 32 points). If Player A wins, he gains 32 minus 22 points, or 10 points, and has
a new rating of 1460, while player B loses 10 points and has a new rating
of 1310. On the other hand, if player B wins then he gains 32 minus
10 points, or 22 points, and has a new rating of 1342, while player A
loses 22 points and has a new rating of 1428. If the game is drawn, then player A “gains” 16 minus 22 points, i.e.
he loses 6 points, and his new rating is 1444, while player B gains 16
minus 10 points, or 6 points, and has a new rating of 1326. In situations where a player is provisionally rated, the rating adjustment
is handled differently. The games of provisionally rated players are
held in a storage file until they achieve 10 results against players
with established ratings. At that time, a pseudorating (for purposes
of the formulae only) is assigned based on their score in those 10 games
and the ratings of their opponents, and then those 10 games are rated
using the formulae above in the ordinary way. Thus, the first rating published for a player is neither their provisional rating assigned for tournament qualification purposes (which is never used in the rating system), nor their pseudorating based on a self-consistency calculation for their 10-game performance and their opponents’ ratings, but rather it is the result of updating the pseudorating using the ordinary ratings update formulae and the 10 (or more) games that have been held for their initial rating. |
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