Mixed ballot transferable vote
The mixed ballot transferable vote refers to a type of vote linkage-based mixed-member electoral system where a group of members are elected on local tier, for example in single-member districts. Other members are elected on a compensatory national tier from a list and voters cast a single ballot where they may indicate their preferences separately.
A dual vote mixed system is not necessarily a mixed ballot system, particularly the ones using separate ballots for the two votes. This article is primarily about systems using mixed ballots. For the dual vote, hybrid versions of parallel voting and MSV used in Hungary and formerly used in Italy for national elections, see scorporo.
Overview
Unused votes from the lower tier are counted on the upper tier in a compensatory way using a vote transfer mechanism. This tied, preferential nature of the dual ballot makes it different from mixed-member proportional and parallel voting systems, which also use two votes for the two tiers. How proportional the outcome is depends on among other factors, the rules and parameters used in the system.In Hungary, elections to the National Assembly use a dual vote based positive vote transfer system, where votes for candidates that did not win a seat are added along with excess votes for the winner in the seat to the list votes. This has plurality SMDs and also partially compensates winning candidates, however, that system uses a parallel voting component to count list votes.
A system formerly used in Italy, scorporo is also a dual vote and vote linkage based mixed electoral system, but differs from MBTV in that it uses negative vote transfer as its compensatory mechanism compensation. These systems lack the "transferable" part of MBTV, in favour of an automatic vote transfer based on party affiliation.
Variations
Plurality
The simplest type of MBTV system allows two single choices on the ballot: one for a local candidate, one for a party list. This type of ballot is also used in the electoral system of Germany and New Zealand.Ranking
A ballot can allow either the voter to rank the candidates or the party lists or both, in this case rankings can be used for a system of elimination, like that of instant-runoff voting. If ranking of both is allowed, the ballot design can also allow to intermix the rankings, providing a full ranking.The ranked ballots can also be used for elections by STV on the local tier, an example is a proposal by Schulz, where not only can voters rank parties intermixed with candidates on a local level, but another party vote can be cast for the MMP-type of compensation. The system utilizes the mixed ballot by providing that if a party has received overhang seats, then only a certain share of those ballots which have contributed to the election candidates of that party will be considered when counting the party votes. The rest of those ballots are considered to have been cast for the party of those candidates who got elected, which avoids split ticket voters double voting power.