Editing Mixed-member proportional representation (section)

===Dealing with overhang seats===
{{see also|Overhang seat}}
If a party wins more FPTP district seats than the proportional quota received by the party-list vote, these surplus seats are called [[overhang seat]]s ({{Lang|de|Überhangmandate}} in German), which may be an obstacle to achieving proportionality.

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 seat]]s to be kept by those candidates who earned it in the constituency elections. 

A counter-example is the ''[[Bundestag]]'' in Germany, where constituency winners may not always keep their seats in accordance with the latest modification of Germany's electoral law. 

In the MMP variant used in Romania in the [[2008 Romanian legislative election|2008]] and [[2012 Romanian legislative election|2012 legislative elections]], constituency seats were only earned by the leading candidate if the candidate also achieved an absolute majority of votes in their district, thereby preventing overhang seats.

In [[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 entitled it to 3 seats in the House but won 5 constituency seats, leaving an overhang of 2 seats. This was compensated for giving two additional seats to other parties, which resulted in a 122-member house. If the constituency seats won had been in proportion to the party vote for the Māori Party, there would have been a normal 120-member house.

To combat disproportionalities caused by overhang seats in most German states, [[leveling seats]] (''Ausgleichsmandate'' in German) are added to compensate for overhang seats and thereby achieve proportionality. Usually 50 percent of total seats are compensatory seats, but that proportion varies. For example, in the provincial parliament (''[[Landtag]]'') of North Rhine Westphalia, 29% of the seats are levelling seats, which compensate for difference between district results based on local votes and the party's share of the party vote. More may be added to balance overhangs. If a party wins more local seats than its proportion of the total party vote justifies, the size of the ''Landtag'' increases so that the total outcome is proportional to the party 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 votes and party votes are combined to determine the proportional allocation of seats.

[[Scottish Parliament|Scotland]] uses a modified variant of MMP known as the [[Additional Member System (UK)|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 representation|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. Wales is similar.) 

For example, 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, (Neither devolved parliament uses a table, instead using a sequential method.) MMP's compensatory effect 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 mixed-member majoritarian. Because there is no provision for overhang seats, there have been cases in Scotland where a party ended up with more seats and others with fewer total seats than their proportional entitlement. This occurred, for example, in the [[South Wales East (Senedd Cymru electoral region)|South East Wales electoral region]] in 2007 and 2016. In 2007 [[Welsh Conservatives]] were under-represented while Independents got one more seat than they were due. In 2016 in that same electoral region, [[Welsh Labour]] was over-represented, while [[Plaid Cymru]] was under-represented. 

Welsh Labour has also been over-represented on this basis in every election in the [[South Wales West (Senedd Cymru electoral region)|South Wales West]] region, and every election in the [[South Wales Central (Senedd Cymru electoral region)|South Wales Central]] region, apart from the 2003 election. This situation arose because Labour held an overwhelming majority of constituency seats in these regions, more than its due share proportionally. Only around one-third of the total number of seats are top-up, in the form of additional regional seats, so that is insufficient to fully compensate for Welsh Labor's over-representation
{| class="wikitable"
! colspan="4" rowspan="3" |
! colspan="2" rowspan="2" |[[Parallel voting]]
([[Mixed-member majoritarian representation|MMM]])
! colspan="6" |Broadly mixed-member proportional type of system (MMP)
|-
! colspan="2" |Additional member system (AMS)
! colspan="2" |Overhang seats re-added
! colspan="2" |True MMP (with leveling seats)
|-
| colspan="2" |[[File:AMS_example_parallel_total_seats.svg|frameless]]
| colspan="2" |[[File:AMS_total_seats.svg|frameless]]
| colspan="2" |[[File:AMS_example_MMP_overhang_only_total_seats.svg|frameless]]
| colspan="2" |[[File:AMS_example_MMP_total_seats.svg|frameless]]
|-
! colspan="2" |Party
!Popular vote (%)
!Constitu&shy;encies won
!Seats
!Share (%)
!Seats
!Share (%)
!Seats
!Share (%)
!Seats
!Share (%)
|-
| style="background:#D10000" |
|Party A
|43%
|54
|67 (54+13)
|67%
|54 (54+0)
|54%
|54 (54+0+0)
|48%
|71 (54+0+17)
|43%
|-
| style="background:#0008A5" |
|Party B
|41%
|11
|24 (11+13)
|24%
|34 (11+23)
|34%
|41 (11+23+7)
|36%
|68 (11+23+34)
|41%
|-
| style="background:#03AA00" |
|Party C
|13%
|0
|3 (0+3)
|3%
|7 (0+7)
|7%
|13 (0+7+6)
|12%
|21 (0+7+14)
|13%
|-
| style="background:#820084" |
|Party D
|3%
|5
|5 (5+0)
|5%
|5 (5+0)
|5%
|5 (5+0+0)
|4%
|5 (5+0+0)
|3%
|-
|
|TOTAL
|100%
|70
|100 (70+30)
|100%
|100 (70+30)
|100%
|113 (70+30+13)
|100%
|165 (70+30+65)
|100%
|-
| colspan="4" |Index of disproportionality
([[Gallagher index|Gallagher]])
| colspan="2" |22.01 (disproportional)
| colspan="2" |10.25 (moderately disproportional)
| colspan="2" |4.97 (considered proportional)
| colspan="2" |0.25 (highly proportional)
|-
| colspan="4" |'''Method used'''
| colspan="2" |Independent PR tier
| colspan="2" |Fixed number of compensatory seats
| colspan="2" |Number of (extra) leveling seats =
number of overhang seats
| colspan="2" |As many leveling seats as needed
|-
| colspan="4" |'''This type of system used in'''
| colspan="2" |Russia, among others
| colspan="2" |Scotland, among others
| colspan="2" |New Zealand, Germany (until 2009)
| colspan="2" |Germany (2013, 2017)
|}