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How would PR have changed the face of Parliament?: Single Transferable Vote

How it works: Long advocated by the Electoral Reform Society and the Liberal Democrats, but in use only in the Irish Republic, Malta and Estonia. It proposes a multi-member constituency, usually of three to five seats: the more seats, the fairer it becomes. On the ballot paper, voters rank candidates in order of preference. To allocate seats, a 'quota' is first fixed by dividing the number of votes by the number of seats plus one. First preferences are counted, and the candidate with the most votes above the quota figure is selected. Any surplus votes of this MP are redistributed. Then the second MP is selected on the same system, and his or her surplus votes redistributed. The process continues until all candidates with votes above the quota level are elected. Then the STV system switches to Alternative Vote - the bottom candidate is eliminated and his or her secondary votes distributed. The process continues until all the seats are filled.

For: Gives a good degree of proportionality without resorting to 'list' MPs. Aids minority parties. The only system tested by the LSE that gave the Greens any MPs.

Against: Complicated, and the hardest system for the voter to try to ensure a desired result. STV constituencies would be much larger, and the system may give too much weight to less favoured preferences.

----------------------------------------------------------------------- How it would have changed the Commons ----------------------------------------------------------------------- Con Lab Lib Dem SNP/PC Green MPs 256 (39.3%) 250 (38.4%) 102 (15.7%) 20 (3.1%) 6 (0.9%) -----------------------------------------------------------------------