The Additional Member System
The Additional Member System is a form of proportional representation. AMS is used for elections to the Scottish and Welsh Parliaments, as well as for those to the London Assembly.
Voters get two ballot papers and may vote on both: one for their constituency representative, elected on the First-past-the-post method, and one for a regional representative. They don’t have to vote for the same party on both ballots. For example, a voter in Paisley in Scotland will receive a ballot that lists all of the candidates wanting to represent Paisley in the Scottish Parliament. They will also receive a ballot that lists the parties and/or independent candidates contesting the seven seats for West Scotland. The candidates on the regional ballot paper are ranked by the political party in the order they want them elected, and voters get no choice over this order (this is known as a 'closed list').
Seat allocation
The overall number of constituency seats that a party wins will have an effect on how many regional representatives it is allocated. This is designed to bring a party's number of seats closer to the percentage of the overall vote it received in a region. Regional seats are allocated according to a formula known as the D’Hondt method. The number of votes needed to win a seat (the ‘quota’) is calculated using the following formula:
Vote quota = Total number of votes won by the party / Total number of seats already won by the party, +1.
Seats are allocated after rounds of counting. In order to be allocated a regional seat, a party must have the highest vote number after the above formula has been applied.
Example election
In the 2016 West Scotland region, the result for the major parties was as follows (with vote quota totals for subsequent rounds shown in brackets):
| Party | Constituency seats won (10 total) | Votes won | First round quota | Second round quota | Third round quota | Fourth round quota | Fifth round quota | Sixth round quota | Seventh round quota | Regional Seats Won |
|---|---|---|---|---|---|---|---|---|---|---|
| SNP | 8 | 135,827 | 15,091 | No change | No change | No change | No change | No change | No change | 0 |
| Labour | 1 | 72,544 | 36,272 (elected) | 24,181 | 18,136 | No change (elected) | No change | No change (elected) | No change | 3 |
| Conservative | 1 | 71,528 | 35,764 | No change (elected) | 23,842 (elected) | 17,882 | No change elected) | 3,576 | No change | 3 |
| Green | 0 | 17,218 | No change | No change | No change | No change | No change | No change | No change (elected) | 1 |
| Lib. Dem. | 0 | 12,097 | No change | No change | No change | No change | No change | No change | No change | 0 |
Although the SNP won the most votes, because they won eight out of the ten constituency seats their quota was 15,092, below almost all the other parties. As a result, they were not entitled to any regional seats.
Labour and the Conservatives also each won a constituency seat, so their vote tally is divided by two in round one of the seat allocation, while everyone else’s is divided by one. Labour’s vote quota in round one was 36,272 (72,544/2) so the top-ranked person on their list was elected.
This process was then repeated, with Labour’s vote total now 24,181 as it was divided by three. The Conservatives, therefore, took the second list seat. This process went on until the seventh seat was won by the Green party, who had 17,218 votes to the SNP’s quota of 15,091.