Editing Additional-member system (section)

==== Compared to similar systems ====
If the 30 additional seats in the example were allocated independently by list-PR the system would be called [[parallel voting]] or a ''supplementary member'' system. This would be a [[Mixed-member majoritarian representation|mixed-member majoritarian]] system (MMM), under which even party A received additional seats, even though it is overrepresented even without getting any.

Some systems called [[Mixed-member proportional representation|mixed-member proportional systems]] (MMP), like the ones used for electing the national parliament [[New Zealand]], at least partially compensate for [[overhang seats]] as well, by adding back that many seats to the assembly if needed, but this is not a perfect correction for the disproportionality. In [[Germany]] formerly even [[Leveling seat|more seats]] were adding to the Bundestag, to get fully proportional results, but as per the latest reform, parties simply may not keep overhang seats, meaning they might not be able to keep all constituency seats they "won" in.

In this example, the assembly size would be increased by 13 seats to compensate for parties B and C's seat deficits under the New Zealand type 'MMP', and by 65 (which allows parties A, B and C to receive more seats) under a flexible amount of additional leveling seats.

The additional member system might provide [[proportional representation]] when there are no overhang seats that would need to be compensated; in this case it would have the same outcome as other 'MMP' systems, if the results of the FPTP elections were completely proportional (which is almost never the case in reality). If decoy lists and tactical voting were used (see below), the results under the AMS would be the same as under parallel voting.

In all other cases the AMS is more proportional than parallel voting, but sometimes less proportional than 'MMP' in New Zealand.
{| class="wikitable"
! colspan="4" rowspan="3" |
! colspan="2" rowspan="2" |Constituency seats only
([[First-past-the-post voting|FPTP]])
! colspan="2" |[[Mixed-member majoritarian representation|Mixed-member majoritarian]]
! colspan="6" |Broadly [[Mixed-member proportional representation|mixed-member proportional]] type of system (MMP)
|-
! colspan="2" |[[Parallel voting]] 
(supplementary member system)
! colspan="2" |Additional member system (AMS)
! colspan="2" | Overhang seats re-added
! colspan="2" | True MMP (with leveling seats)
|-
| colspan="2" |[[File:AMS fptp seats.svg|frameless]]
| 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 (%)
!Seats
!Share (%)
|-
| style="background:#D10000" |
|Party A
|43%
|54
|54
|77%
|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
|11
|16%
|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
|0
|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
|7%
|5 (5+0)
|5%
|5 (5+0)
|5%
|5 (5+0+0)
|4%
|5 (5+0+0)
|3%
|-
|
|TOTAL
|100%
|70
|70
|100%
|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" |31.55 (highly disproportional)
| 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" |Only first-past-the-post
| 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" |United Kingdom, among others
| colspan="2" |Russia, among others
| colspan="2" |Scotland, London
| colspan="2" |New Zealand, Germany (until 2009)
| colspan="2" |Germany (2013, 2017)
|}