Arpad Elo suggested that a 200-pt rating difference on the ELO chess scale should mean that the higher-rated player's expected score in a game should be 0.75 and the lower-rated player should expect 0.25. Not sure that translates to a probability of the poorer player winning of 1 in 4. The appropriate cumulative distribution or logistic distribution curve can be consulted to see what the expectations are for larger Elo differences. Elo ratings are calculated on the basis of competitive results. In contrast, my work with Giuseppe Di Fatta and Ken Regan ranks players on the basis of their choice of moves. q.v. http://centaur.reading.ac.uk/view/creators/90000763.default.html and in particular ... (working backwards) ... http://centaur.reading.ac.uk/23800/, http://centaur.reading.ac.uk/19778/, http://centaur.reading.ac.uk/4517/, ... Maybe worth mentioning at this point the concept of 'Fischer Distance': Anyone who has beaten Fischer at chess has Fischer-distance 1 Anyone who has beaten someone who has beaten Fischer has Fischer distance 2 (or less), etc. Guy Haworth