Ratings - Background
Each player is allocated a provisional rating which ranges from 300 (Junior Development Division) to 2,000 (Division 1).
Rating points are gained or lost depending on the rating difference between the two players.
In the Sevenoaks League the following matrix is used:
For example, suppose A had a rating of 1500 and loses to B who had a rating of 1800. The rating difference is 300 and the result is an Expected Win for B and an Expected Loss for A.
So B is awarded 3 points and moves up to 1803 while A loses 3 points and moves down to 1497.
If A had beaten B then this would have been an unexpected win for A and an unexpected loss for B.
In this case A gains 45 points and B loses 45 points.
In some leagues the points deducted for a loss are less than those awarded for a win which leads to a general increase in ratings year on year. Table Tennis England ratings are reduced by 10% at the end of each season as this serves to bring players closer together without changing the rating order!
The determination of an initial rating can be a problem. Sometimes we have to make an educated guess and then refine it as results come in.
If a player plays in another league which operates a ratings system then plotting players' ratings who play in both leagues usually produces a graph as follows. By assuming a straight line through the origin we can get a best fit such as y=0.6676x. Then substituting the player's known rating (x) gets an estimate of his rating (y).
Bear in mind that the slope of the line will vary from year to year.