On the interpretation of "off the edge" avalanches
  We establish both experimentally and theoretically the relation between off the edge and internal avalanches in a sandpile model, a central issue in the interpretation of most experiments in these systems. In BTW simulations and also in the experiments the size distributions of internal avalanches show power laws and critical exponents related with the dimension of the system. We show that, in a SOC scenario, the distributions of off the edge avalanches do not show power laws but follow scaling relations with critical exponents different from their analogous for the internal avalanche distributions. 
O. Ramos, A.J. Batista-Leyva and E. Altshuler