. "Flows of knowledge from universities and federal laboratories: Modeling the flow of patent citations over time and across…." (NAS Colloquium) Science, Technology and the Economy. Washington, DC: The National Academies Press, 1996.
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Proceedings of the National Academy of Sciences of the United States of America
FIG. 3. Frequency of citations to U.S. patents, from patents originating in the United States, the European Economic Community, Canada, and Japan. The localization effect fades over time.
The “fading” effect in the geographic dimension corresponds to the intuitive notion that knowledge eventually diffuses evenly across geographic and other boundaries, and that any initial “local” advantage in that sense will eventually dissipate. Once again, these results offer a quantitative idea of the extent of the initial localization and the speed of fading. Notice also that starting a few years after grant, the differences across regions seem to depend upon a metric of geographic, and perhaps also cultural, proximity: at lag 10, for example, Canada is highest with a coefficient of 0.67, followed by Europe with 0.53, and Japan with 0.44.
Further Results. Finally, the overall estimate of β1=0.2 means that the citation function reaches its maximum at about 5 years, which is consistent with the empirical citation distribution shown in Fig. 1. The R2 of 0.52 is fairly high for models of this kind, suggesting that the postulated double exponential combined with the effects that we have identified fit the data reasonably well.
The computerization of patent citations data provides an exciting opportunity to examine the links among inventions and inventors, over time, space, technology, and institutions. The ability to look at very large numbers of patents and citations allows us to begin to interpret overall citation flows in ways that better reflect reality. This paper represents an initial exploration of these data. Many variations that we have not explored are possible, but this initial foray provides some intriguing results. First, we confirm our earlier results on the geographic localization of citations, but now provide a much more compelling picture of the process of diffusion of citations around the world over time. Second, we find that federal government patents are cited significantly less than corporate patents, although they do have somewhat greater “staying power” over time. Third, we confirm our earlier findings regarding the importance or fertility of university patents.
Table 5. Citation probability ratio by citing geographic area
Rest of World
Interestingly, we do not find that university patents are, to any significant extent, more likely to be cited after long periods of time. Finally, we show that citation patterns across technological fields conform to prior beliefs about the pace of innovation and the significance of “gestation” lags in different areas, with Electronics, Optics, and Nuclear Technology showing very high early citation but rapid obsolescence, whereas Drugs and Medical Technology generate significant citations for a very long time.
The list of additional questions that could be examined with these data and this kind of model is even longer. (i) It would be interesting to examine if the geographic localization differs across the corporate, university, and federal cited samples. (ii) The interpretation that we give to the geographic results could be strengthened by examining patents granted in the United States to foreign corporations. Our interpretation suggests that the lower citation rate for foreign inventors should not hold for this group of cited patents. (iii) We could apply a similar model to geographic regions within the United States, although some experimentation will be necessary to determine how small such regions can be and still yield reasonably large numbers of citations in each cell while controlling for other effects, (iv) It would be useful to confirm the robustness of these results to finer technological distinctions, although our previous work with citations data lead us to believe that this will not make a big difference, (v) We would like to investigate the feasibility of modeling obsolescence as a function of accumulated patents. Caballero and Jaffe (7) implemented this approach, but in that analysis patents were not distinguished by location or technological field.
We acknowledge research support from National Science Foundation Grants SBR-9320973 and SBR-9413099.
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