chessmec: <quote>Sonneborn-Berger [...] is an usual used system.</quote>

yes, it is, but in my eyes it's nonsense. In a round robin you may look at the result(s) between tied players but any other system is just an arbitrary way of breaking ties at all costs.

SB basically honours wins against higher ranked opponents. Agreed: if a player has a better SB then he has scored better against the stronger opponents. But he has also lost more points against the weaker opponents, so what's the sense in granting him the higher rank?