Scotland uses an electoral system called the Additional Member System (AMS) and is similar to the one used in Germany and New Zealand. It combines constituency seats with regional seats to create a parliament which both broadly reflects the views of the electorate whilst giving each voter a designated MSP.

How to vote

Voters are given two ballot papers, one Lilac and one Peach.

• The Lilac ballot paper is for the constituency race, place an X beside a single Candidate.
• The Peach ballot is for the list race, place an X beside a single Party.

Counting the votes

Scotland has 73 constituency seats, each seat is awarded to whichever party wins the most votes in that constituency. There is no need to win a majority of the votes, the highest number wins. This is known as First Past the Post and tends to produce seat numbers which do not reflect vote share but gives every constituency a designated MSP.

Once all the constituency seats are awarded the system begins to distribute the regional list seats. There are 8 regions and 7 seats are given out in each.

List seats are given to parties which didn’t win enough seats in the constituencies. This is done by the D’Hondt formula - this adds 1 to the number of constituency seats within that region to create their ‘divisor’. Each party’s vote share is then divided by their divisor.

The party with the highest vote share after the division wins the first list seat, and 1 is added to their divisor. Vote shares are divided by the divisors again, and a new seat awarded.

This is repeated until all 7 seats are awarded. As they go to parties which are underrepresented in the constituencies this creates a much more proportional result.

In order to win a single list seat a party needs to win roughly 6% of the vote in a region. It means that several parties will enter parliament but the difference in vote shares between those winning the final list seats and those missing out can be under 1%.

Strategic Voting

It is possible to use your vote in a way which maximizes its likelihood of flipping a seat. The most common method of doing so is to vote for a large party which could win in your constituency, even if they aren’t your first choice.

On the list, it can be better to vote for a party which struggles to win constituency seats but which can win over the 5-6% effective threshold on the list ballot. This is especially powerful if your favoured party is likely to win more constituency seats than its list share would suggest. For a more thorough understanding of how this has previously occurred in Scotland see here.

The Projection

The Projection uses proportional swing on constituency ballot for every party other than Reform, Alba and the Greens as they did not contest most constituencies. It uses the same measure on the list for every party, other than Reform - as Reform only received 0.2% last.

For example: The Liberal Democrats won 5.1% of the overall list vote in 2021. If the user input was to increase that to 10.2% the model doubles the Liberal Democrats vote share in every region.

For the Greens, Reform and Alba, a more complex model has been developed which distributes unassigned votes in proportion to the user’s projection vote share - with regional variation built in.

Northern Ireland uses an electoral system called Single Transferable Vote (STV) which is similar to the one used in Ireland, Malta and Scottish local elections. It allows voters to rank candidates in order of preference to pick five constituency MLAs, the resulting Assembly both broadly reflects the views of the electorate.

How to vote

Voters are given one ballot paper.

Voters rank candidates in order of preference. 1 to the candidate they like the most, 2 whoever they like second most, 3 for third, 4 for 4th etc…

They can rank every candidate or just a few, it's their choice. As a general rule, rank as many as you can stomach.

Counting the votes

There are 18 constituencies each with five seats, creating 90 MLAs.

In each constituency, seats are given to candidates who reach the Quota. The Quota is determined by D’Hondt formula, but as we have 5 seats in every constituency it is always 16.7% of the vote.

First we count all number 1 preferences. If a candidate wins more than 16.7%, the extra votes (‘surplus’) are transferred to other candidates according to who that candidate’s voters ranked as their next highest preference.

If no one reaches 16.7%, we eliminate whoever is in last place. Those votes are then given to their voters' next highest preferences.

This continues until 5 candidates reach the quota, or only 5 candidates are left.

The Projection

The Projection uses proportional swing at the constituency level.

For example: The UUP won 11.2% of the overall vote in 2022. If the user input was to increase that to 22.4% the model would double the UUP vote share in every constituency.

Modelling Transfers

Transfers are decided by voters when they rank candidates in order of preference. Generally, transfers go to like minded candidates, especially to candidates of the same party.

For example, if all major parties stood a single candidate:

The model expects most voters of the TUV to give their second preference to the DUP, whilst some would prefer the UUP. Although some will only vote for the TUV and no else (Non-transferable). If fellow unionists aren’t an option at that stage (because they were elected or eliminated), some TUV voters will give a lower preference to Alliance or even the SDLP, but very few will give Sinn Féin a preference.

The model distributes TUV transfers amongst unionist parties and non-transferable first, with most going to the DUP. If there are no other unionists left in the race then Alliance will receive some TUV transfers, the SDLP a couple, Sinn Féin wouldn’t get any. But most are non-transferable at this stage.

This model does the same for every party. This is calculated dynamically depending on which parties are left in the constituency race at the point that votes are being distributed in an attempt to mimic real-life transfer patterns as closely as possible.

Wales uses Closed List-PR. This is a very common system across Europe, being used in Spain, Portugal and even here in the UK. All European Parliamentary elections from 1999-2019 used this system (excluding Northern Ireland). Voters pick their preferred party and seats are given out proportionally, the order of candidates is decided by the parties themselves.

How to vote

Voters are given one ballot paper.

Place an X beside the party you like most.

Counting the votes

The new Senedd will have 96 seats. 16 constituencies will elect 6 candidates, the 16 constituencies use the same boundaries as the 32 used at Westminster, except two neighbouring constituencies have been combined.

Seats are given out via the D’Hondt formula, each party's vote share is divided by the number of seats they currently have in each constituency, +1.

Every party starts with a divisor 1 (this isn’t a real seat and is just to avoid divide by zero errors). The first round is therefore each party’s vote shares divided by 1, which is just their vote shares again. The highest number gets the first seat and 1 is added to their divisor.

All party vote shares are divided by their new divisor, this is 1 for everyone except the party that won the first seat whose number is divided by 2. The largest after this gets the second seat and 1 added to their divisor.

This continues until 6 seats are awarded.

The cut off is officially 14.3% in each constituency, but this is higher than it will actually be.

In reality parties are likely to win seats with around 11-12% because smaller parties and spread out votes make the numbers less perfect.

Strategic Voting

It is very difficult to vote strategically in Senedd elections under the new system. It can be better to vote for a party that you think is going to be close to the cut off for the final seat, but this is a risky strategy.

The Projection

The baselines were created by Dr Jac Larner, a lecturer in Politics at Cardiff University. We have slightly amended them to better fit the methodology of this projection. The projection uses proportional swing, this ensures that parties increase more in areas where they’re already strong, which is in keeping with real-world trends. It ensures that an increase in user input for Plaid Cymru will result in greater change in the northwest than in eastern Wales. For example: The Liberal Democrats won about 4.4% of the vote in 2021, so if the user increases that to 8.8% the model doubles the Liberal Democrats vote share in every region, but ensures that it never exceeds 100% and is smoothed out across Wales.