  A recent econometric literature has critiqued the use of regression
discontinuities where administrative borders serves as the 'cutoff'.
Identification in this context is difficult since multiple treatments can
change at the cutoff and individuals can easily sort on either side of the
border. This note extends the difference-in-discontinuities framework discussed
in Grembi et. al. (2016) to a geographic setting. The paper formalizes the
identifying assumptions in this context which will allow for the removal of
time-invariant sorting and compound-treatments similar to the
difference-in-differences methodology.
