Fitting loglinear Bradley-Terry models (LLBT) for paired comparisons using the R package prefmod

This paper aims at introducing the R package prefmod (Hatzinger, 2009) which allows the user to
fit various models to paired comparison data. These models give estimated overall rankings of objects
or items where each subject (respondent/judge) makes one or more comparisons between pairs of
objects (items). The focus is on the loglinear Bradley-Terry (LLBT) model, the loglinear formulation of
the Bradley-Terry(-Luce) model, both assuming independence between comparisons. Five types of data
structures are covered: (i) simple paired comparisons, (ii) paired comparisons including an undecided
category, (iii) categorical subject covariates (for estimating different overall rankings for different
subject groups), (iv) object covariates for reparameterizing objects, and (v) order (position) effects.
Additionally, the discussion briefly addresses other response formats such as ratings and (partial)
rankings.