Semi-proportional representation

The choice to use a semi-proportional electoral system may be a deliberate attempt to find a balance between single-party rule and proportional representation.

Election systems in which a party can achieve its due share of seats (proportionality) only by coordinating its voters are usually considered to be semi-proportional.

Many writers consider the single transferable vote to be a semi-proportional system because of its substantial favoritism towards major parties, generally caused by a combination of the Droop quota in small districts, as well as the substantial degree of vote management involved when there are exhausted ballots.

Similarly, the 1998 Northern Ireland elections resulted in the Ulster Unionists winning more seats than the Social Democratic and Labour Party with a smaller share of the vote.

This caused a constitutional crisis, leading to a provision to provide bonus seats in case of disproportional results.

The Hare quota is theoretically unbiased, allowing some of the errors in apportionment to cancel out if voters across the whole country.

However, it also increases the vulnerability of STV to vote management by large parties, allowing them to win the same number of seats they would have won under Droop.

Looking to the electoral systems effectively in use around the world, there are three general methods to reinforce the majoritarian principle of representation (but not necessarily majoritarianism or majority rule, see electoral inversion and plurality) starting from basic PR mechanisms: parallel voting, the majority bonus system (MBS), and extremely reduced constituency magnitude.

When this imbalance is created intentionally, the result could be described as a semi-proportional system — for example, in the National Assembly for Wales, where only 33.3% of members are compensatory.

Majority bonuses help produce landslide victories similar to those which occur in elections under plurality systems.