Exploring Heterogeneity In Irish Voting Data : A Mixture Modelling Approach ∗

Irish elections use a voting system called proportional representation by means of a single transferable vote (PR-STV). Under this system, the voters express their votes by ranking some (or all) of the candidates in order of preference. Which candidates are elected is determined through a series of counts where candidates are eliminated and surpluses are distributed. The electorate in any election forms a heterogeneous population. Voters with different political and ideological persuasions would be expected to have different preferences for the candidates. Mixture models are proposed to explore the heterogeneity amongst the voters of the Irish electorate. The proposed mixture model describes the electorate as a finite collection of homogeneous populations and a ranking data model is assumed within each of these homogeneous populations. The application of mixture models to study voter heterogeneity is demonstrated on data from two Irish elections. Data from eight opinion polls taken during the six weeks prior to the 1997 Irish presidential election are analyzed. These data reveal the evolution of the structure of the electorate during the election campaign. In addition, data that record the votes from the Dublin West constituency of the 2002 Irish general election are analyzed to reveal heterogeneity in the actual votes in Irish elections.

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