Tournaments are often seeded to provide an even distribution of strong and weak competitors across all preliminary groups. However, in association football, the ranking methods used for seeding may be crude. In the World Cup, until 2018 the usual strategy was for each group to contain one seeded team and three unseeded teams, with the unseeded teams picked from separate regional confederations. Some North American, African and Asian teams are significantly stronger than others. The net result was that some groups may have had stronger teams than others. From the 2018 edition onwards this system has been changed, with a distribution of teams based on the FIFA World Rankings introduced, but with continental limitations still to be retained.