Author: Dieter Buerssner
Date: 11:41:25 04/26/05
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Eelco, did you compare the numbers calculated with the approximation formula to those given in the FIDE handbook? There was a larger difference than what you have given as the error, when I tried it. I used a very accurate version of erf() by Stephen Moshier (I think, 15 digits correct). I used the following formulas: p = 0.5+0.5*erf(dp/400) and p = 1/(1+10^(-dp/400)) p is the expected score, dp the rating difference. IIRC, USCF uses the later formula based on the logistic distribution. In the center (rating difference small), both formulas give practically identical values. In the tails, they differ (not much in absolute terms, but significantly taken into relation). The numbers given on the FIDE site where in between. Regards, Dieter
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