[EM] In effect and in voting theory, Plurality is a rankings method. (Supplementary Vote)

[James Gilmour]

Chris wrote:> 
> By way of a contrasting example, there is a horrible method 
> called "the Supplementary Vote" (is or was used to elect the 
> Lord Mayor of London in the UK), which is a version of IRV 
> that restricts voters to voting a single first and a single 
> second preference. Obviously, with four or more candidates, this is 
> not equivalent to IRV because it can give a different result.

The "Supplementary Vote" is indeed a horrible voting system, but I don't think it is helpful to
describe it as a version of IRV.  I think it may pre-date IRV (= ballot papers marked with unlimited
preferences and candidates eliminated one-at-a-time) and it is perhaps better seen as a different
branch of development from run-off elections.  It was used in Australia from 1892 to 1942, when it
was known as the "Contingent Vote".  It was also used in the Democrat primaries in Alabama from 1915
to 1931.  More recently, it has been used to elect the President of Sri Lanka since 1982 (voters can
mark up to 3 preferences, but the 'run off' is between only the top two candidates).  Regrettably
this horrible voting system was adopted by the UK Labour Government for the election of the Mayor of
London and for the mayors of the small number of other towns and cities in England that have opted
for this model of local government.

In the UK version voters can mark only two preferences but do not mark conventional preferences
("1", "2").  Instead they mark "X" against one candidate in the first column and "X" against one
(different) candidate in the second column.  If no candidate has an absolute majority of the first
preference votes, all but the first two candidates are eliminated and the votes for the eliminated
candidates transferred.  Second preferences not for one of the leading two candidates are thus
discarded.  In the May 2000 election for the London Mayor, these discarded votes were 57% of the
potentially transferable votes.  Chris did say it was a horrible system!
James