  We propose a generic model for multiple choice situations in the presence of
herding and compare it with recent empirical results from a Web-based music
market experiment. The model predicts a phase transition between a weak
imitation phase and a strong imitation, `fashion' phase, where choices are
driven by peer pressure and the ranking of individual preferences is strongly
distorted at the aggregate level. The model can be calibrated to reproduce the
main experimental results of Salganik et al. (Science, 311, pp. 854-856
(2006)); we show in particular that the value of the social influence parameter
can be estimated from the data. In one of the experimental situation, this
value is found to be close to the critical value of the model.
