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87 N.C. L. Rev. 1657 (2008-2009)
Patent Citation Networks Revisited: Signs of a Twenty-First Century Change

handle is hein.journals/nclr87 and id is 1667 raw text is: PATENT CITATION NETWORKS REVISITED:
SIGNS OF A TWENTY-FIRST CENTURY
CHANGET
KATHERINE J. STRANDBURG, GABOR CSARDI, JAN TOBOCHNIK,
PtTER tRDI, AND LASZLO ZALANYI**
This Article reports an empirical study of the network composed
of patent nodes and citation links between them. It builds
on an earlier study in which we argued that trends in the growth
of the patent citation network provide evidence that the explosive
growth in patenting in the late twentieth-century was due at least
in part to the issuance of increasingly trivial patents. We defined
a measure of patent stratification based on comparative
probability of citation; an increase in this measure suggests that
the United States Patent and Trademark Office is issuing patents
of comparatively less technological significance. Provocatively,
we found that stratification increased in the 1990s during the
patent explosion. Here we report a further study indicating
that the trend toward increasing stratification leveled off
beginning around 2000. This observation suggests that there was
a de facto tightening of patentability standards well before the
doctrinal shifts reflected in the Supreme Court's flurry of patent
activity  beginning  around   2005.     We also    investigate the
possibility that changes in our measure of stratification are due to
* Copyright © 2009 by Katherine J. Strandburg, Gfbor Csfrdi, Jan Tobochnik, Peter
Erdi, and L~szl6 Zal6nyi.
** Katherine J. Strandburg is Professor, DePaul University College of Law and was
Visiting Professor, Fordham University School of Law (Fall 2008). G~ibor Cs~rdi is
currently a postdoctoral researcher in the Department of Medical Genetics, University of
Lausanne, Switzerland. Much of this work was completed while he was a doctoral student
at the Department of Biophysics, KFKI Research Institute for Particle and Nuclear
Physics of the Hungarian Academy of Sciences, Budapest, Hungary, and Center for
Complex Systems Studies, Kalamazoo College. Jan Tobochnik is Dow Distinguished
Professor of Natural Science, Department of Physics, Kalamazoo College. Pter lrdi is
Henry Luce Professor, Departments of Physics and Psychology, Kalamazoo College, and
Head, Department of Biophysics, KFKI Research Institute for Particle and Nuclear
Physics of the Hungarian Academy of Sciences, Budapest, Hungary. Liszl6 Zaldnyi is a
doctoral student, Department of Biophysics, KFKI Research Institute for Particle and
Nuclear Physics of the Hungarian Academy of Sciences, Budapest, Hungary, and Center
for Complex Systems Studies, Kalamazoo College. We are grateful to the American
Institute of Physics for providing the data we used in the study described in Part II.D.1 of
this Article.

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