Compensation commonly occurs either through
seat linkage compensation (or top-up) or through
vote linkage compensation (or vote transfer). Like a non-compensatory mixed system, a compensatory mixed system may be based on the
mixed single vote (voters vote for a local candidate and that vote is used to set the party share of the popular vote for the party that the candidate belongs to) or it may be based on voters casting two separate votes.
Example of seat linkage compensation In both mixed
compensatory systems and mixed
non-compensatory systems, two sets of seats are allocated using different methods. Most often, this involves one
winner-take-all system, usually
first-preference plurality. The remaining seats are allocated to political parties partially or wholly based on a proportional allocation method such as
highest averages or
largest remainder. The difference is whether or not the results of the district elections are considered when allocating the PR seats. In mixed non-compensatory systems, such as
parallel voting, the proportional allocation is performed independently of the district election component. In seat linkage mixed compensatory systems, the allocation of the top-up seats is done in such a way as to compensate as much as possible for dis-proportionality produced by the district elections. MMP generally produces proportional election outcomes, meaning that a
political party that wins
n percent of the vote will receive roughly
n percent of the seats. The following hypothetical example based on the one by Massicotte The Scandinavian countries have a long history of using both multi-member districts (members elected through party-list PR) and nationally based compensatory top-up seats using the same method as MMP, however because the local MPs are also elected using PR, these systems are not usually considered MMP as they are not
mixed systems.
Dealing with overhang seats When a party wins more constituency seats than it would be entitled to from its proportion of (party list) votes, most systems allow for these
overhang seats to be kept by those candidates who earned it in the constituency elections. A counter-example would be the MMP variant used in Romania in the
2008 and
2012 legislative elections, where constituency seats could only be earned by the winning candidate if they also achieved an absolute majority in their district, thereby eliminating overhang seats. In Germany's
Bundestag and the
New Zealand House of Representatives, all members elected for constituencies keep their seats. For example, in the
2008 New Zealand general election the
Māori Party won 2.4% of the party vote, which would entitle them to 3 seats in the House, but won 5 constituency seats, leaving an overhang of 2 seats, which resulted in a 122-member house. If the party vote for the Māori Party had been more in proportion with the constituency seats won, there would have been a normal 120-member house. In most German states, and in the federal
Bundestag since 2013, the other parties receive extra seats (leveling seats) to create full proportionality. For example, the provincial parliament (
Landtag) of North Rhine Westphalia has, instead of the usual 50% compensatory seats, only 29% unless more are needed to balance overhangs. If a party wins more local seats than its proportion of the total vote justifies, the size of the
Landtag increases so that the total outcome is fully proportional to the votes, with other parties receiving additional list seats to achieve proportionality. The leveling seats are added to the normal number of seats for the duration of the electoral period. In the German state of
Bavaria, the constituency vote and party vote are combined to determine the distribution of seats.
Scotland and
Wales use a modified variant of MMP known as the
additional-member system where due to the nature of the calculations used to distribute the regional list seats, overhang seats are not possible; the list allocation works like a
mixed-member majoritarian system, but in using the
d'Hondt method's divisors to find the averages for the allocation, the first divisor for each party takes into account the number of constituency seats won by the party; i.e. a party that won 7 constituency seats would start with a divisor of 8 (7 seats + 1 per the method's divisor formula) instead of 1. The resulting table would then give 7 seats for Scotland and 4 seats for Wales to the parties possessing the highest averages on the table, although both devolved parliaments do not use a table, instead using a sequential method. The compensatory effect characteristic of MMP is in the fact that a party that won constituency seats would have lower averages on the table than it would if the election used MMM. Because of no provision for overhang seats, there have been cases where a party ended up with fewer total seats than its proportional entitlement. This occurred, for example, in the elections in the
South East Wales electoral region in both 2007 (
Welsh Conservatives under-represented) and in 2016 (
Welsh Labour over-represented,
Plaid Cymru under-represented). Labour has also been over-represented on this basis in every election in the
South Wales West region, and every election in the
South Wales Central region apart from the 2003 election. This situation arises because Labour has continued to hold the overwhelming majority of constituency seats in these regions, and only around one-third of the total number of seats are available for distribution as additional regional seats.
Threshold Vote linkage between tiers == Tactical manipulation ==