A total number of fictional voters can then be established and the party strength can be deduced. The number of deputies in each party is determined at the cantonal level using proportional representation with the Hagenbach-Bischoff system (except in single-member cantons.) The election's turnout is computed as: number of valid ballots cast / number of registered voters. Role The role and powers of the National Council are regulated by the (The Federal Law on the Federal Parliament (Parliament-Law)) and the fifth article of the Swiss Federal Constitution. The National Council, together with the Council of States, forms the Federal Parliament and exercises the highest legal authority in Switzerland, subject to the rights of the people and the cantons. Both chambers of the Federal Parliament are called "councils" (Räte). The National Council and the Council of States do not meet daily, but meet regularly for sessions. Usually, there are four sessions in a year, each lasting three weeks, with between two and five sittings per week. The spring session (Frühjahrssession) begins on the first Monday in March, the summer session (Sommersession) on the first Monday in June, the Autumn session (Herbstsession) after the Federal Day, and the winter session (Wintersession) on the last Monday in November. During the sessions, proposed legislation is debated. If there is not enough time in the regular sessions, an extra session can be convened. In special situations (political crises, wars, etc.) a quarter of the members of one of the two councils or the Federal Council can convene an extraordinary session. To date, there have been eight extraordinary sessions, most of them called by the social democratic parliamentary group. Powers The National Council and the Council of States are constitutionally completely equal - a bill is only law when it has been accepted by both councils in the same version. All business is considered by both councils in turn. The presidents of the councils decide together which council will handle a given matter first (Erstrat) Sometimes, after the first reading, the National Council and the Council of States end up producing different texts, in which case a difference resolution procedure takes place, in which the bill is sent back and forth between the two councils. After a bill has been sent back three successive times, the two councils must meet together to discuss the matter. Each year the National Council elects a , who leads sessions