  In this paper, we analyze the response of music and book sales to an external
field and to buyer herding. We distinguish endogenous and exogenous shocks. We
focus on some case studies, whose data have been collected from ranking on
amazon.com. We show that an ensemble of equivalent systems quantitatively
respond in a similar way to a similar ''external shock'', indicating roads to
universality features. In contrast to Sornette et al. [Phys. Rev. Lett. {93},
228701 (2004)] who seemed to find power law behaviors, in particular at long
times, - a law interpreted in terms of an epidemic activity, we observe that
the relaxation process can be as well seen as an exponential one that saturates
toward an asymptotic state, itself different from the pre-shock state. By
studying an ensemble of 111 shocks, on books or records, we show that exogenous
and endogenous shocks are discriminated by their short-time behaviour: the
relaxation time seems to be twice shorter in endogenous shocks than in
exogenous ones. We interpret the finding through a simple thermodynamic model
with a dissipative force.
