A single-member district (SMD) or single-member constituency is an electoral district that returns one officeholder to a body with multiple members such as a legislature. This is also sometimes called single-winner voting or winner takes all. The alternatives are multi-member districts, or the election of a body by the whole electorate voting as one constituency (i.e. proportional representation).
Many single member systems can be run independently in districts to form a Regional System.
There are two two key factors in single member representation: how the members are chosen (the voting method) and how the districts are drawn.
In most SMD systems, districts are divided by units of population, by administrative subregions, or other metrics to equalize representation (in theory).
The process of changing the SMD boundaries is called redistricting. Changing districts for partisan gain is called gerrymandering.
Popular Single Member systems
Single-winner voting methods can be sub-classified by different ways to aggregate the ballots.
- Plurality Voting: A valid vote can choose only one candidate
- Ordinal Voting: A valid vote can rank candidates 1,2,3... (Tied rankings are permitted in some methods but not others)
- Tied rankings not permitted
- Tied rankings permitted
- Condorcet-compliant methods
- VOTE-123: another name for Condorcet methods, stands for Virtual One-on-one Tournament Elections using 1st, 2nd, & 3rd choices
- Majority voting or Maximum Majority voting: another term often used for Condorcet methods
- Bucklin voting: approval with virtual runoff; each voters' ballot is counted for more candidates each round until some candidate reaches a majority
- Cardinal Voting: voting A valid vote allows independent numerical values to be associated with each candidate. (The set of valid values is limited.)
They can also be classified on how many times votes can be counted. Methods like Plurality, Borda, and Approval with single counting rounds are simpler since voters can be sure to know how their votes will be applied.
Single Winner Variations
- Automatic Equal Ranking Line Option (AERLO)
- A voter may mark a line in his/her ranking, meaning that if no one above that line wins, then that voter wants to promote to 1st place all of his/her above-line candidates and have a recount. (In pairwise-count methods the promotion only takes place if, additionally, there's a circular tie containing above-line and below-line candidates).
- Automatic Truncation Line Option (ATLO)
- A voter may mark a line in his/her ranking, meaning that if no one above the line wins, then that voter wants to drop from his/her ranking all of his/her below-line candidates and have a recount. (In pairwise-count methods the dropping only takes place if, additionally, there's a circular tie containing above-line and below-line candidates).