I’ve heard it’s a standardized scoring method for many more competitions than just chess. But what is it and how does it work?
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It’s just a simple calculation. Everyone starts with a score, and the score can be increased or decreased depending on the results of a match.
The expected outcome is EA=1/(1+10^((B-A)^)/400 )
Where EA is the expected outcome for A, and B is B’s score and A is A’s score.
EA + EB =1 always where EB is the expected outcome for B
Let S be the outcome of a game. A win counts at 1, a loss counts at 0, and a draw is .5
Let’s calculate A’s new score after the match.
A’ = A + K(S-EA)
K is a linear adjustment and can be tampered with. Usually it’s 16 for masters and 32 for weaker players, but in most games it will be the same for both parties.
So if EA was .75, and A won the game, A’s score will increase
Similarly, B’s will decrease.
If instead, B won the game, A’s score will decrease by a lot, and B’s will increase by a lot thanks to that S-EA term (or S-EB in B’s case) 1-.25 is much bigger than 1-.75 and 0-.25 is a smaller loss than 0-.75
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