Proportional Representation (PR) is a type of voting where the number of seats on the governing body that each political party gets are determined by the percentage of votes that each party receives. In most PR systems, parties submit lists of candidates and voters vote either for a party or for individuals representing a party. Each party receives a number of seats directly proportional to the percentage of votes they received in the election. In most PR systems, the individual parties determine what members get the available seats each election. In some systems regional lists are used and seats are allocated separately in each region. PR systems tend to give representation to several parties, thereby making coalition government or minority government more likely.
Types of Proportional Representation
Voters either cast a single vote for a party or rank parties in order of preference. Seats are then allocated according to the party lists. Closed list systems are common in Europe.
Voters may either vote for a party list or may substitute their own preferences. Open list systems are common in the Nordic countries and the Netherlands
Mixed Member Proportional (MMP)
Voters vote for individual candidates in single districts and for a prefered ruling party for all districts. The support for parties is determined either by adding the votes for individual candidates or from a separate party vote, then "top-up" seats are allocated at a regional or national level to achieve a more proportional result. Mixed member systems are used in Germany, Italy and New Zealand where a party has to reach a 5% threshold in order to get into the House.
Single Transferable Vote (STV)
Single Transferable Vote (STV) is not strictly a proportional system although in practice it results in more proportional results than plurality voting. It allow voters to vote for the individual candidates in their constituency according to preference. Rather than ticking the box next to the desired candidate and ignoring the others, Irish voters can technically vote for every candidate on the ballot in order of preference - 1 to 10, or more, according to how many candidates there are. According to their resources, parties field as many candidates in as many constituencies as possible.
The population of the constituency determines how many seats in the legislature are available (in Ireland, 2, 3, 4 or 5) and a 'quota' is set, which is the target number of 'No.1' votes a candidate must reach to be elected. Once a candidate reaches the quota, his/her surplus No.1 votes are redistributed according to the preferences expressed on the ballot papers. This process is repeated until all the available seats are filled. Candidates who are found to be mathematically incapable of reaching the quota during each redistribution are eliminated.
Parties attempt to use this system to maximise their representation in each constituency - bigger parties run several candidates in each constituency, and a 'superstar' candidate who knows s/he'll get large support will encourage her/his supporters to give their second, third etc preference to colleagues on the party ticket. For example, this contributor's constituency, the 5-seat Dublin South Central, is represented in the following way: 2 Fianna Fáil, 1 Fine Gael, 1 Labour and 1 Sinn Féin.