The terminology of electoral system reform can be confusing: the same systems are often called by different names, in part due to developments in different nations and in part due to tactical decisions by reformers. Here are some of the key terms used in our article. -- R.R. and S.H.
List System. In this most widely used form of PR, the voter votes for one party and its list of candidates to represent them. Party lists of candidates can be either "closed" or "open." A closed list means that parties determine the order of their candidates to be elected, often by primary or caucus. An open list allows voters to determine a party's list of candidates by indicating preferences for individual candidates. If a party wins 30 percent of the vote, its candidates win roughly 30 percent of the seats in the legislature, 10 percent of the vote wins roughly 10 percent of seats, and so on. Nationwide lists are used in some countries, but most have regional lists in smaller constituencies. A minimum share of the votes can be required to earn representation; Germany has a 5 percent threshold.
Mixed Member Proportional (MMP). Increasingly popular around the world, this hybrid system elects some seats from single-seat, "winner-take-all" districts and some from party lists. MMP combines geographic representation and proportional representation of ideological interests. Depending on how party lists are allocated, MMP can be fully proportional or semi-proportional.
Choice Voting. Also known as "single transferable vote" and "preference voting," choice voting is the most common form of PR in English-speaking nations. Despite being based on voting for candidates rather than parties, it allows blocs of like-minded voters to win proportional representation. Each voter has one vote, but can rank candidates in order of preference (1, 2, 3, etc.), and ranking additional candidates has no impact on a higher choice candidate's chance to win. Candidates win by reaching a "victory threshold" that is roughly the number of votes divided by the number of seats. If a candidate has too little support among first choices to win, votes for that candidate are transferred to voters' next choices.
Instant Runoff Voting (IRV). Also known as "alternative vote" and "majority preference" voting, IRV is based on the same "transferable vote" mechanism as choice voting, but is a "winner-take-all" system for electing a single candidate such as president, mayor, or governor. Each voter has one vote, but can rank candidates in order of preference (e.g., 1-Nader, 2-Perot, 3-Clinton). The ballot count simulates a series of runoff elections. The candidate with the fewest first-place votes is eliminated, and ballots cast for that candidate are "transferred" to second choices as indicated on voters' ballots. This process of transferring votes continues until one of the candidates has a majority.
Cumulative Voting. A semi-proportional system used in some American localities. Voters have as many votes as seats elected in their constituency, and can allocate them however they wish--including giving more than one vote to a particular candidate. Cumulative voting is semi-proportional because votes can be "wasted" if a candidate receives more than necessary to win, or if two or more candidates "split" the vote of a particular constituency.
For more thorough discussion of these and other forms of PR, see Matthew Shugart and Rein Taagepera, Seats and Votes (New Haven: Yale University Press, 1989); Douglas Amy, Real Choices, New Voices (New York: Columbia University Press, 1993); and the Institute for Democracy and Electoral Assistance's Handbook of Electoral System Design.
Originally published in the February/ March 1998 issue of Boston Review