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[H.G.Muller]

OK, I see what you mean. By adding more parameters, one can always get a better fit. I was just looking for the best possible model that only takes rating difference into account.

I agree that in general predictions should better not be tested on the data they are derived from, because it will make you err into the direction of thinking that they are better than they really are. But for a very large data set it hardly matters. (E.g. the N/(N-1) correction you need for variances computed to a mean derived from the points itself, or from an independently given one.)

In that light, what do you think of the fact that the data points seem to stipulate a steeper-rising curve than the green logistic on which they are supposed to be based? Is this an indication that BayesElo's default approach is not the optimal way to extract the ratings?