Clustering ranked preference data using sociodemographic covariates

Ranked preference data arise when a set of judges rank, in order of their preference, a set of objects. Such data arise in preferential voting systems and market
research surveys. Covariate data associated with the judges are also often recorded.
Such covariate data should be used in conjunction with preference data when drawing inferences about judges.
To cluster a population of judges, the population is modelled as a collection
of homogeneous groups. The Plackett-Luce model for ranked data is employed to
model a judge’s ranked preferences within a group. A mixture of Plackett-Luce
models is employed to model the population of judges, where each component in
the mixture represents a group of judges.
Mixture of experts models provide a framework in which covariates are included
in mixture models. Covariates are included through the mixing proportions and
the component density parameters. A mixture of experts model for ranked preference data is developed by combining a mixture of experts model and a mixture of
Plackett-Luce models. Particular attention is given to the manner in which covariates enter the model. The mixing proportions and group specific parameters are potentially dependent on covariates. Model selection procedures are employed to
choose optimal models.
Model parameters are estimated via the ‘EMM algorithm’, a hybrid of the
Expectation-Maximization and the Minorization-Maximization algorithms. Examples are provided through a menu survey and through Irish election data. Results
indicate mixture modelling using covariates is insightful when examining a population of judges who express preferences.