Single-member district

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A single-member district 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.

Many single member systems can be run independently in districts to form a Regional System. The systems:

  • Plurality voting: A valid vote can choose only one candidate
  • Approval voting: A valid vote can only give a yes or nothing to a given candidate.
  • Ordinal voting: A valid vote can rank candidates 1,2,3... (Tied rankings are permitted in some methods but not others)
  • Cardinal voting: voting A valid vote allows independent numerical values to be associated with each candidate. (The set of valid values is limited.)

Classification[edit | edit source]

They can be sub-classified by different ways to aggregate the ballots.

Plurality voting[edit | edit source]

There is only one way to combine plurality votes.

Approval voting and Cardinal voting[edit | edit source]

Since Approval is the degenerate case of Cardinal Ballots they have the same A sum would give the Utilitarian_winner while a median would give the majoritarian winner.

Ordinal voting[edit | edit source]

Borda count and Instant-runoff voting are common aggregation methods


Popular Single Member systems[edit | edit source]

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[edit | edit source]

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).