Ice hockey tournaments have been staged at the Olympic Games since 1920. The men's tournament was introduced at the 1920 Summer Olympics and was transferred permanently to the Winter Olympic Games program in 1924, in France. The women's tournament was first held at the 1998 Winter Olympics. The Olympic Games were originally intended for amateur athletes until 1988, and the National Hockey League did not allow its players to compete until 1998. From 1924 to 1988, the tournament started with a round-robin series of games and ended with the medal round. Medals were awarded based on points accumulated during that round. The games of the tournament follow the rules of the International Ice Hockey Federation, which differ slightly from the rules used in the NHL. The tournament follows the World Anti-Doping Agency's rules on performance enhancing drugs and the IIHF maintains a Registered Testing Pool, a list of top players who are subjected to random in-competition and out-of-competition drug tests. Several players have tested positive for banned substances since the 1972 Winter Olympics.