Table 1. Simple statistics for patent subsamples

 

United States corporations

United States universities

United States government

Range of cited patents

1963–1990

1965–1990

1963–1990

Range of citing patents

1977–1993

1977–1993

1977–1993

Total potentially cited patents

88,257

(1 in 10)

10,761

(Universe)

38,254

(Universe)

Total citations

321,326

48,806

109,729

Mean citations

3.6

4.5

2.9

Mean cited year

1973

1979

1973

Mean citing year

1986

1987

1986

Cited patents by field, %

Drugs and medical

4.89

29.12

3.36

Chemicals excluding drugs

30.37

28.71

20.73

Electronics, optics, and nuclear

26.16

27.39

45.40

Mechanical

28.18

9.51

17.09

Other

10.39

5.28

13.42

Citations by region, %

United States

70.6

71.8

70.8

Canada

1.6

1.7

1.7

European Economic Community

14.5

13.2

16.8

Japan

11.3

11.0

8.6

Rest of world

1.9

2.4

2.1

ent potentially citing locations differ in the speed and extent to which they “pick up” existing knowledge, as evidenced by their acknowledgment of such existing knowledge through citation. Because of the policy context mentioned above, we are particularly interested in citations to university and government patents. We recognize that much of the research that goes on at both universities and government laboratories never results in patents, and presumably has impacts that cannot be traced via our patent citations-based research. We believe, however, that at least with respect to relatively near-term economic impacts, patents and their citations are at least a useful window into the otherwise “black box” of the spread of scientific and technical knowledge.

The analysis in this paper is based on the citations made to three distinct sets of “potentially cited” patents. The first set is a 1-in-10 random sample of all patents granted between 1963 and 1990 and assigned to United States corporations (88,257 patents). The second set is the universe of all patents granted between 1965 and 1990 to United States universities, based on a set of assignees identified by the Patent Office as being universities or related entities such as teaching hospitals (10,761 patents).f The third set is the universe of patents granted between 1963 and 1990 to the United States government (38,254 patents). Based on comparisons with numbers published by the National Science Foundation, these patents are overwhelmingly coming from federal laboratories, and the bulk come from the large federal laboratories. The United States government set also includes, however, small numbers of patents from diverse parts of the federal government. We have identified all patents granted between 1977 and 1993, which cite any of the patents in these three sets (479,861 citing patents). Thus we are using temporal, institutional, geographic, and technological information on over 600,000 patents over about 30 years.

Some simple statistics from these data are presented in Table 1. On average, university patents are more highly cited, despite the fact that more of them are recent.g Federal patents are less highly cited than corporate patents. But it is difficult to know how to interpret these averages, because many different effects all contribute to these means. First, the differences in timing are important because we know from other work that the overall rate of citation has been rising over time (7), so more recent patents will tend to be more highly cited than older ones. Second, there are significant differences in the composition of the different groups by technical field. Most dramatically, university patents are much more highly concentrated in Drugs and Medical Technology and less concentrated in Mechanical Technology, than the other groups. Conversely, the federal patents are much more concentrated in Electronics, Optics, and Nuclear Technology than either of the other groups, with less focus on Chemicals. To the extent that citation practices vary across fields, differences in citation intensities by type of institution could be due to field effects. Finally, different potentially citing locations have different field focuses of their own, with Japan more likely to cite Electronics patents and less likely to cite Drug and Medical patents. The main contribution of this paper is the exploration of an empirical framework in which all of these different effects can be sorted out, at least in principle.

The Model

We seek a flexible descriptive model of the random processes underlying the generation of citations, which will allow us to estimate parameters of the diffusion process while controlling for variations over time and technological fields in the “propensity to cite.” For this purpose we adapt the formulation of Caballero and Jaffe (7), in which the likelihood that any particular patent K granted in year T will cite some particular patent k granted in year t is assumed to be determined by the combination of an exponential process by which knowledge diffuses and a second exponential process by which knowledge becomes obsolete. That is:

p(k, K)=α(k, K) exp[–β1(k, K)(T – t)]

× [1–exp(–β2 (T–t))],

[1]

where β1 determines the rate of obsolescence and β2 determines the rate of diffusion. We refer to the likelihood determined by Eq. 1 as the “citation frequency,” and the citation frequency as a function of the citation lag (T—t) as a citation

f  

There are, presumably, university patents before 1965, but we do not have the ability to identify them as such.

g  

In previous work (6), we showed that university patents applied for up until about 1982 were more highly cited than corporate patents, but that the difference has since disappeared.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement