Marco, AC (Marco, Alan C.) The dynamics of patent citations ECONOMICS LETTERS, 94 (2): 290-296 FEB 2007

[Eugene Garfield]

E-mail Address: marco at vassar.edu 

Author(s): Marco, AC (Marco, Alan C.) 

Title: The dynamics of patent citations 

Source: ECONOMICS LETTERS, 94 (2): 290-296 FEB 2007 

Language: English 

Document Type: Article 

Author Keywords: patents; patent citations; hazard rate; duration 

Keywords Plus: COMPETITION; SPILLOVERS; KNOWLEDGE 

Abstract: The use of patent citations as a measure of patent "quality" 
increased dramatically in recent years. I estimate the hazard of patent 
citation, and find evidence of unobserved heterogeneity. Hazard estimation 
provides a means to separate patent quality from citation "inflation". (c) 
2006 Elsevier B.V. All rights reserved. 

Conclusion: This note shows that there is unobserved heterogeneity in the 
rate of patent citation, and that
individual hazards exhibit positive duration dependence despite the 
average hump-shaped hazard rate
over the age of the patent. The results highlight the need for care in 
interpreting the marginal effects of
forward citations on other variables of interest, such as firm value. At 
the least they must be interpreted
in light of the fact that the citation rate changes over the life of a 
patent; at best, one would want to
model citation dynamics. An alternative would be to include as regressors 
those factors that influence
the rate of forward citations, e.g., the industry or technology group, the 
year of patent grant, or the
observed life of the patent. That approach has two limitations. First, it 
does not account for the clear
non-linearity in citation rates. Second, it does not allow the researcher 
to separately identify direct
effects of patent citations from the independent effects of other 
regressors, such as industry or
technology.
Researchers have expressed concern that citation inflation may bias the 
estimated effect of citations on
other variables of interest. Indeed, it is difficult to separate the 
effects of inflation from those of patent
quality. If researchers are able to ascertain those factors that are 
correlated to citation inflation rather than
quality, then hazard estimation suggests the rendering of new metrics for 
patent quality.
By estimating a hazard rate based only on inflationary factors, residuals 
can be used to measure latent
patent quality. For example, the difference between actual citation lags 
and predicted citation lags may be
a better measure of patent quality than simple citation counts. 
Alternatively, the ratio of observed citations
to predicted citations may represent a proxy of patent quality. The 
advantage of such constructs is that
they can be calculated as time-invariant values that are not subject to 
censoring.
Of course, the proposed methods for measuring patent quality are 
predicated upon a correlation
between unobserved heterogeneity in patent citation and patent quality. 
This is an area left for future
research.

Addresses: Vassar Coll, Dept Econ, Poughkeepsie, NY 12604 USA 

Reprint Address: Marco, AC, Vassar Coll, Dept Econ, 124 Raymond Ave 592, 
Poughkeepsie, NY 12604 USA. 

E-mail Address: marco at vassar.edu 

Cited Reference Count: 9 

Times Cited: 0 

Publisher: ELSEVIER SCIENCE SA 

Publisher Address: PO BOX 564, 1001 LAUSANNE, SWITZERLAND 

ISSN: 0165-1765 

29-char Source Abbrev.: ECON LETT 

ISO Source Abbrev.: Econ. Lett. 

Source Item Page Count: 7 

Subject Category: Economics 

ISI Document Delivery No.: 140GK 

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