  In order to disentangle the internal dynamics from exogenous factors within
the Autoregressive Conditional Duration (ACD) model, we present an effective
measure of endogeneity. Inspired from the Hawkes model, this measure is defined
as the average fraction of events that are triggered due to internal feedback
mechanisms within the total population. We provide a direct comparison of the
Hawkes and ACD models based on numerical simulations and show that our
effective measure of endogeneity for the ACD can be mapped onto the "branching
ratio" of the Hawkes model.
