  In this paper we analyze the US Patient Referral Network (also called the
Shared Patient Network) and various subnetworks for the years 2009--2015. In
these networks two physicians are linked if a patient encounters both of them
within a specified time-interval, according to the data made available by the
Centers for Medicare and Medicaid Services. We find power law distributions on
most state-level data as well as a core-periphery structure. On a national and
state level, we discover a so-called small-world structure as well as a
"gravity law" of the type found in some large-scale economic networks. Some
physicians play the role of hubs for interstate referral. Strong correlations
between certain network statistics with healthcare system statistics at both
the state and national levels are discovered. The patterns in the referral
network evinced using several statistical analyses involving key metrics
derived from the network illustrate the potential for using network analysis to
provide new insights into the healthcare system and opportunities or mechanisms
for catalyzing improvements.
