London’s Mayoral elections: maxmising my voice by strategic use of preference voting
by Jack Thurston
I want Ken Livingstone to be re-elected as Mayor of London. I believe that as a politician he is a cut above any of the other candidates and has the experience and record of achievement that entitle him to a third term of office. I have even made a small donation of £50 to his campaign. But I have today cast my (postal) vote for the Green candidate. Why?
Well, unusually for the UK, where we are used to a first-past-the-post system in our Parliamentary elections, the London mayoral election is based on a voting system known as ‘batch-style alternative vote’, one of the family of single transferable vote systems, also known as preference voting systems.
This is how it works. Every elector gets two votes - a first preference and a second preference. All the first preferences are counted and if no candidate has received more than 50 per cent of all votes cast, there is a second round. In the second round, all but the top two candidates from the first round are eliminated and the votes cast for the eliminated candidates are redistributed to the remaining two, on the basis of second preferences. The winner is the candidate with the largest combined tally of first and redistributed second preferences. If that was hard to follow, there is a short video explaining it here or in pictures here.
I want to maximise my voice in this election and so I am making strategic use of preference voting. While I want Ken Livingstone to be elected, I also want to express my support for the environment, sustainability and quality of life agenda advocated by Sian Berry, the Green Party candidate. While I care about these issues, I do not want the Green Party candidate to be elected as Mayor.
I am under no illusions that the only two candidates who have any chance of winning are Ken Livingstone (Labour) and Boris Johnson (Conservative). These will be the candidates who contest the second round. This is why I’m casting my first preference for the Green candidate and my second preference for Ken Livingstone. In so doing, I hope to re-elect Ken (via the redistribution of my second preference vote to him following the elimination of Sian Berry) and at the same time send a message to him that I really care about sustainability issues. I hope that Ken Livingstone in his third term as Mayor will see that the Green candidate received a significant minority of votes: votes like mine which were subsequently redistributed to him. I want him to know that I support him but I want him to pursue a sustainability agenda in office.
Could this strategic use of a preference vote backfire? I don’t think so. All the polls show that this is a two-horse race, and so the chance of Ken Livingstone being eliminated in the first round are close to zero. Of course, this assumes that only a small proportion of voters will vote strategically like me. If everyone voted strategically, then it could go terribly wrong. For example, if Ken Livingstone received 90% of strategically-cast second preferences but was eliminated in the first round because he did not get enough first preferences. I think this is highly unlikely.
Comments
Is the redistribution the transfer of a complete vote? You don’t transfer fractions as in some STV systems?
Yes, in the second round there is no difference in weight between a first preference vote and a redistributed second preference vote.
Fractional transfer is pretty rare, certainly for mass elections. I believe it is used more for elections with a rather smaller electorate (say a University board or such like).
OK, good…
I’ve never been involved with the technicalities of an AV election, only STV elections, and there you have a variety of weights for the re-distributions (although they don’t do that in Ireland).
This explains it:
http://pl.atyp.us/misc/votefaq.txt
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Single Transferable Vote (STV)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This is a system for electing multiple candidates in a constituency.
Each voter marks ‘1′ against the candidate they most like, ‘2′ against
their next favourite, and so on until they have no preference for the
remaining candidates.
The highest preferences for each candidate are counted. If any
candidate has more votes than the Quota, they are elected. The Quota
is calculated as:
Quota = int(V/(N 1)) 1
where:
V = number of votes
N = number of candidates to be elected
int() = convert to integer, rounding down
The rationale for this formula is that the quota is the smallest
number of votes such that the candidate is certain to be within
the top N candidates.
If someone is above the quota, the additional votes above the quota
are reallocated to the other candidates according to the voters’
lower preferences. This is done by giving each vote of the candidate
a fractional value according to the fraction that the candidate was
above the quota.
(In Ireland they use the simpler, but less precise, system of simply
picking a bunch of the candidate’s ballot papers at random and using
those for reallocation.)
If no-one is above the quota, the candidate with the smallest number
of votes is eliminated, and their votes are reallocated to other
candidates.
When votes are reallocated, if the voter has expressed no further
lower preferences, their vote is discarded.
This procedure is continued until all N candidates are elected.