Issues Affecting The Single Transferable Vote - District Size

District Size

Another issue commonly considered with STV elections is the size of the voting districts in terms of the number of candidates elected and, to a lesser extent, the total size of the body being elected.

Because STV is proportional, larger districts reduce the support a candidate requires to become elected as a percentage of the district. With 9 to be elected, for example, any who reach (with transfers) 10% electoral support will win a seat, whereas with 5 to be elected 16.7% is required. Some STV elections make use of districts with the number of seats available as small as three. A larger number of candidates elected also results in a smaller number of wasted votes on the final count. However, larger districts and the implicit larger number of candidates also increase the difficulty of giving meaningful rankings to all candidates from the perspective of the individual voter, and may result in increased numbers of exhausted ballots and reliance on party labels or group voting tickets.

While Ireland originally had a median district magnitude of five (range three to nine) in 1923, successive governments lowered this. A parliamentary committee in 2010 discussed the "increasing trend towards the creation of three-seat constituencies in Ireland." They found "a larger district magnitude is shown to have positive effects in relation to the representation of women and minorities, as well as allowing for a plurality of ideas in terms of policy." They recommended not less than four-seaters, except where the geographic size of such a constituency would be disproportionately large. When Northern Ireland adopted STV, they found five-seaters not sufficiently proportional and chose six-seaters. Tasmania had seven-seaters until the Green Party won the balance of power in 1996, after which the two large parties shrank the district magnitude to five and called an early election which cut the Green representation from four seats down to one.

Larger electoral districts can also significantly reduce the effects of gerrymandering; because gerrymandering relies on wasted votes to award the "last seat" in each district, proportional representation systems such as STV with larger multimember districts are intrinsically more difficult to gerrymander. Larger districts can also make for significantly harder tactical voting: since the problem of making correct assumptions about other voter's behavior and rearranging one's tactical ballot is NP-hard, the difficulty of tactical voting increases sharply as the number of candidates grows.

There is no theoretical upper limit to the size of districts in STV, and they may not even be needed at all: Thomas Hare's original proposal was for a single, nationwide district. In theory, STV could allow for the election of particularly small minorities provided they secure a quota's worth of votes, if very large districts were used. No jurisdiction currently uses districts larger than nine for election to its main legislature, although the 1925 Irish Senate election used one district to elect nineteen places.

A related question is the number of people in the district, and the possibility of an effective democratic campaign by a candidate in a large district where campaign costs are high. In Northern Ireland districts range from 96,000 people in six-seaters for the Northern Ireland Assembly, and even smaller districts municipally, to a single three-seater district of all 1,737,000 people for election to the European Parliament.