Originally Posted by Litotes
You're thinkin 12 as quotient, I see. This could conceivably be higher for important tournaments and lower for unimportant ones.
It will take a long time for rankings to show a true picture if a victory over Federer initially is worth no more than a victory over a 16-year old WC in his first ATP match.
Not really, (+/-)12 is not the quotient here, but just the result of the calculation, and it reflects the difference in player's "strength". In the case that both players have the same rating (points), it is 12, but say Federer had 2890 points, and Tomic had 2000, then if Federer beat Tomic, he would gain 24 points, and Tomic would loose the same. But if Tomic beat Federer, then they would both gain 0 points, that is, they would both retain their previous points. And if we imagine that there could be a draw in tennis match, then Federer would still gain 12 points from the draw, and Tomic would loose 12 points. That is simply how calculations in Elo system work. And the point is not to rate tournaments as important or unimportant, the point is that wining is awarded the same way on any tournament (between the same players).
The difference in points reflects probability that one player would win over the other, something like determining the betting odds. If a player with higher ranking wins lower ranking player, firstly, it is actually expected, and secondly, by wining, that player increases his chances of winning again, and that is reflected on increased difference in points. Rating system is in a way cumulative.
What I would like to discuss, is how to grade a result of a match.
In chess, there are 2 outcomes, win/loose and draw, and win is awarded 1, draw 0.5, and loosing 0. In Elo rating calculations, result should map to a range between 0 and 1. In tennis, there can be only win/loose, so simple way would be to award 1 for win, and 0 for loosing. But, I am thinking of a finer grading, that could reflect more subtle differences between player's performances, and also establish the difference between matches on grand slams and matches on other tournaments (winning in 3 sets vs. winning in 2 sets).