Tutorial 1: Our first electoral system

Let’s start defining real elections that are plain and simple.

UK - General election (Caerfyrddin)

In the UK General election, a Parliament member is elected from each constituency using the first-past-the-post system.

For out example, we will define the Caerfyrddin constituency (2014 General election).

We will use the YAML format for electoral systems definitions:

1. Create a district and set an identifier:

   name: Caerfyrddin constituency (2014 UK General election)
   

2. Add the election method (first-past-the-post). This method can be accessed by two keys: first_past_the_post and winner_takes_all.

   name: Caerfyrddin constituency (2014 UK General election)
   method: first_past_the_post
   

3. Add votes for candidates:

   name: Caerfyrddin constituency (2014 UK General election)
   method: first_past_the_post
   candidates:
   - name: DAVIES, Ann
     alliance: PC
     votes: 15520
   - name: O'NEIL, Martha
     alliance: Lab
     votes: 10985
   - name: HART, Simon
     alliance: Con
     votes: 8825
   - name: HOLTON, Bernard
     alliance: RUK
     votes: 6944
   - name: BECKETT, Nick
     alliance: LD
     votes: 1461
   - name: BEASLEY, Will
     alliance: Green
     votes: 1371
   - name: COLE, Nancy
     alliance: WEP
     votes: 282
   - name: EVANS, David
     alliance: WPB
     votes: 216
   

4. We can add invalid votes inserting a special candidate whose name starts with !:

   name: Caerfyrddin constituency (2014 UK General election)
   method: first_past_the_post
   candidates:
   - name: "! invalid vote"
     votes: 187
   - name: DAVIES, Ann
     alliance: PC
     votes: 15520
   - name: O'NEIL, Martha
     alliance: Lab
     votes: 10985
   - name: HART, Simon
     alliance: Con
     votes: 8825
   - name: HOLTON, Bernard
     alliance: RUK
     votes: 6944
   - name: BECKETT, Nick
     alliance: LD
     votes: 1461
   - name: BEASLEY, Will
     alliance: Green
     votes: 1371
   - name: COLE, Nancy
     alliance: WEP
     votes: 282
   - name: EVANS, David
     alliance: WPB
     votes: 216
   

   There are two special candidates recognised by a name prefix:

   • Invalid votes, with prefix !.

   • Blank votes (valid empty votes), with prefix ?.

   Candidates can be provided directly from file. That will be covered soon.

5. Save the file as Caerfyrddin_2014.yaml.

Now, this system can be computed using the command interregnum-cli:

interregnum-cli calc Caerfyrddin_2014.yaml results.yaml

Spain - European Parliament

No regional districts are considered for the Spanish EU Parliament elections:

• Seats are apportioned using the highest averages method with d'Hondt divisors.

• In addition to invalid votes, blank votes (valid votes to no candidate) are also considered.

• In the year 2024 61 members of the European Parliament were allocated to Spain.

We can reuse the schema for the UK General election, but with some additions:

name: Spain 2024 - European Parliament
method: highest_averages
method_params:
    divisor_f: dhondt
seats: 61
candidates:
- name: "! invalid vote"
  votes: 124569
- name: "? blank vote"
  votes: 124655
...

1. The HighestAveragesAllocator requires a parameter divisor_f with the name of the divisor function:

   method: highest_averages
   method_params:
       divisor_f: dhondt
   seats: 61
   

2. Since there is a multi-seat allocation, seats need to be provided:

   method_params:
       divisor_f: dhondt
   seats: 61
   candidates:
   

3. Blank vote is added as a special candidate whose name has a prefix ?:

   candidates:
   - name: "! invalid vote"
     votes: 124569
   - name: "? blank vote"
     votes: 124655
   ...
   

Now, all candidates should be included to the schema, but we will omit it for the sake of brevity.

Belgium (2012) - Local elections

Let’s do one more…

In Belgium, the local elections used the same system than the Spain EU Parliament elections, but the Imperiali divisor is used instead.

In 2012 47 seats were allocated to Anderlecht:

name: Commune d'Anderlecht - 2012
method: highest_averages
method_params:
    divisor_f: imperiali
seats: 47
candidates:
- name: "N-VA"
  votes: 2258
- name: "LB"
  votes: 11648
- name: "FDF"
  votes: 3499
- name: "ECOLO-GROEN"
  votes: 5057
- name: "VLAAMS BELANG"
  votes: 1630
- name: "PS-SP.A-CDH"
  votes: 16383
- name: "PTB*PVDA+"
  votes: 755
- name: "GAUCHES COMMUNES"
  votes: 278
- name: "ISLAM"
  votes: 1839
- name: "EGALITE"
  votes: 829
- name: "BELG-UNIE"
  votes: 385