Koya system

The Koya system (KS) looks only at results against the more successful part of the field. The International Chess Federation (FIDE) defines it for round-robin tournaments, where each participant plays every other participant: add the points a player scored against every participant whose final score reached at least 50% of the maximum tournament score.

For example, in a 9-game round robin using 1 point for a win, the maximum score is 9 and the normal Koya threshold is 4.5. Only points scored against opponents who finish on 4.5 or more contribute to Koya. Results against players below that threshold contribute nothing to KS.

This differs from Sonneborn-Berger, which weights each result by the opponent's exact final score. Koya instead sets a threshold and then counts the points scored against players who meet it. FIDE also permits variations through a limit modifier that raises or lowers the threshold in half-point steps.