 The difference in rating between two players determines an estimate for the expected score between them.Both the average and the spread of ratings can be arbitrarily chosen.Elo suggested scaling ratings so that a difference of 200 rating points in chess would mean that the stronger player has an "expected score" (which basically is an expected average score) of approximately 0.75, and the USCF initially aimed for an average club player to have a rating of 1500.