  We unveil collective effects induced by imitation and social pressure by
analyzing data from three different sources: birth rates, sales of cell phones
and the drop of applause in concert halls. We interpret our results within the
framework of the Random Field Ising Model, which is a threshold model for
collective decisions accounting both for agent heterogeneity and social
imitation. Changes of opinion can occur either abruptly or continuously,
depending on the importance of herding effects. The main prediction of the
model is a scaling relation between the height h of the speed of variation peak
and its width $w$ of the form h ~ w^{-kappa}, with kappa = 2/3 for well
connected populations. Our three sets of data are compatible with such a
prediction, with kappa ~ 0.62 for birth rates, kappa ~ 0.71 for cell phones and
kappa ~ 0.64 for clapping. In this last case, we in fact observe that some
clapping samples end discontinuously (w=0), as predicted by the model for
strong enough imitation.
