On the boundary between qualitative and quantitative measures of causal effects
  Causal relationships among variables are commonly represented via directed
acyclic graphs. There are many methods in the literature to quantify the
strength of arrows in a causal acyclic graph. These methods, however, have
undesirable properties when the causal system represented by a directed acyclic
graph is degenerate. In this paper, we characterize a degenerate causal system
using multiplicity of Markov boundaries, and show that in this case, it is
impossible to quantify causal effects in a reasonable fashion. We then propose
algorithms to identify such degenerate scenarios from observed data.
Performance of our algorithms is investigated through synthetic data analysis.
