  We present a study of the group purchasing behavior of daily deals in Groupon
and LivingSocial and introduce a predictive dynamic model of collective
attention for group buying behavior. In our model, the aggregate number of
purchases at a given time comprises two types of processes: random discovery
and social propagation. We find that these processes are very clearly separated
by an inflection point. Using large data sets from both Groupon and
LivingSocial we show how the model is able to predict the success of group
deals as a function of time. We find that Groupon deals are easier to predict
accurately earlier in the deal lifecycle than LivingSocial deals due to the
final number of deal purchases saturating quicker. One possible explanation for
this is that the incentive to socially propagate a deal is based on an
individual threshold in LivingSocial, whereas in Groupon it is based on a
collective threshold, which is reached very early. Furthermore, the personal
benefit of propagating a deal is also greater in LivingSocial.
