Party Lists and Candidate Election Probabilities

One of the major goals of this site is to try and predict election results based on recent relevant political polling. This is intended to include not just the total number of seats won by each party, but also viable coalition possibilities and electorate level results.

Today I present the simulation results at the candidate level, including probabilities for each major or minor party candidate to be elected to parliament by either winning an electorate or being selected off their party list. First though, in the interests of disclosure, I thought I should release the data I’m using for the party lists. The 2008 Party Lists are available in MS Excel (.xlsx) format [82kB].

The simulation requires the party lists to be input in the form of a list linking each candidate to the electorate they stood for (or else indicating they were a list only candidate.) The list is based largely on the information on the Party lists for the 2008 General Election page on the Elections New Zealand website, and the electorate information from the Candidates by electorate page from Wikipedia, and is amended at discretion.

The data format is as follows:

1. Party Code : A unique code for each political party.  Parties are numbered 0~7, and ordered firstly by the number of seats won in the 2008 NZ General Election, and secondly by the number of party votes received.
2. Party
   
3. List Ranking
4. Name : The name of the candidate as given on the Elections New Zealand website.
5. Electorate : The electorate the candidate stood in. For list-only candidates this will read “list only.”
6. Electorate Code : A unique code for the electorate.  Electorates are numbered in alphabetical order, with general electorates (#0 ~ #62) preceding Maori electorates (#63 ~ #69). If the candidate is a list-only candidate candidate this code will take the value -1.

For computational reasons each major or minor party candidate standing in an electorate must have a list ranking. In order to avoid having this requirement affect the results the lists are simply extended to 100 candidates for each party, with electorate-only candidates placed in the lowest ranked positions such that they will never be elected to a list seat. The intermediate positions are then filled with dummy candidates: for example, the candidate “NAT-74-list only” refers to the 74th ranked candidate on the National Party list, with the “list only” suffix indicating that they are a list-only candidate. Please feel free to use or amend the file at will. Corrections gratefully accepted.

The current simulation, however, does not use the 2008 Party Lists. Instead it references an alternative list that has been amended to take into consideration changes during the current term of Parliament. This current list used for the simulation is also available in MS Excel (.xlsx) format [82kB]. A summary of changes is as follows:

1. Labour : The list reflects the retirements of Helen Clark and Michael Cullen during the term of Parliament. Candidates ranked #3 (Phil Goff) onwards are moved up two list places each. Current Parliamentary members are moved up in the list ahead of unsuccessful candidates (ahead of list candidate #42 Judith Tizard). A new candidate replacing Helen Clark (David Shearer, Mount Albert) is included, ranked #41 and inserted into the list ahead of Judith Tizard. A new dummy list candidate (“LAB-77-list only”) is inserted in position #77 in place of Michael Cullen.
2. National : Richard Worth (unsuccessful candidate for Epsom, list rank #23) is removed, and subsequent candidates are moved up one rank. A new dummy candidate for Epsom (“NAT-67-Epsom”) is inserted in position #67 in his place.
3. Green : Jeanette Fitzsimons (list-only candidate, list rank #1) and Sue Bradford (candidate for East Coast Bays, list rank #3) are removed, and subsequent candidates moved up. A new dummy candidate for East Coast Bays (“GRE-66-East Coast Bays”) is inserted in position #66 replacing Sue Bradford. A new dummy list candidate (“GRE-67-list only”) is inserted in position #67 in place of Jeanette Fitzsimons.

The above modifications are only intended to capture the spirit of changes since the beginning of the term of parliament and are of course subject to change when the parties release their official party lists closer to the date of the 2011 General Election. If anybody has any serious objections – or is just curious how things would work out with different party lists – and is willing to provide an updated list in the same format then I would be happy to rerun any simulations.

The candidate level results of the most recent simulation (January 22nd, after the release of the latest Roy Morgan Research poll) are shown in the table below (please click for an enlarged view.)

The table gives the probabilities for each candidate to be elected to Parliament by winning an electorate, by being elected from their party list, and an overall probability for either method combined. The “Rank” column gives the respective candidate’s relative likelihood of being elected, and is ordered firstly by probability to be elected, and then by party code and list ranking where there is a tie. Probabilities are rounded to the nearest percent.

The first 89 ranked candidates are guaranteed to be elected, and will of course win 89 seats between them. Candidates ranked 90 through to 114 are considered highly likely to be elected, and each have individual probabilities in the 90% to 100% range. These 25 candidates are expected to win a further 24.6 seats between them, for a cumulative total of 113.6 seats in Parliament. After this we get to the marginal list and electorate candidates: those ranked 115 through to 135 have probabilities in the 10% to 90% range. These 21 candidates are expected to win a further 9.4 seats between them, for a cumulative total of 123.0 seats in Parliament. Next we have 14 more canadidates ranked 136 through to 149 who are considered highly unlikely to be elected to Parliament, with probabilities of less than 10% each. These 14 candidates are expected to win only 0.2 seats between them, bringing the cumulative total to 123.2 seats – an expected overhang of 3.2 seats. Finally, candidates ranked 150 through 800 (many not shown in the table for ease of viewing) have no chance of being elected to Parliament based on current polling data.

I realise that the above calculations may seem little more than trivia at the moment given that we are so far out from the next election, and that the finalised party lists and electorate candidates will not be known for a long time. However, the main motivation for doing this simulation is a hope that people will be able to see which individual list candidate their party vote is likely to be counted for. Closer to the 2011 General Election I will begin publishing “effective party lists” on a regular basis. These effective lists will show only those candidates on the cusp of winning a list seat for each party, and will hopefully give NZ voters a better idea of where their vote is going, and a more meaningful alternative to a quick glance at the top of the lists for each party which some seem to use now when deciding how to cast their votes.