Identifying the age profile of patent citations: new estimates of knowledge diffusion
指出专利引用年龄分布研究中存在申请年、引用年和专利年龄的共线性问题，提出以申请到授权的时滞作为外生变异的灵活识别策略，并重新检验了不同技术领域和申请年专利的引用年龄分布。
Abstract Previous research studies the age profile of patent citations to learn about knowledge flows over time. However, identification is problematic because of the collinearity between application year, citation year, and patent age. We show empirically that a patent's ‘citation clock’ does not start until it issues, and propose a highly flexible identification strategy that uses the lag between application and grant as a source of exogenous variation. We examine the potential bias if our assumptions are incorrect, and discuss extensions into other research areas. Finally, we use our method to re‐examine prior results on citation age profiles of patents from different technological fields and application year cohorts. Copyright © 2009 John Wiley & Sons, Ltd.