Why Priest-Klein Cannot Apply to Individual Issues in Patent Cases

Jason Rantanen, University of Iowa College of Law


The Priest-Klein hypothesis is commonly used to support the expectation of a fifty-percent success rate for patent holders on issues such as obviousness or inequitable conduct. As this essay explains, such a use of Priest-Klein is mathematically flawed. At best, the Priest-Klein hypothesis only applies to the selection of disputes, not the selection of individual issues. Due to the presence of multiple issues in patent cases, there is axiomatically no basis for inferring that a patentee would expect a fifty-percent chance of winning on each one. This essay supports its argument with theory and examples, demonstrating why application of the Priest-Klein hypothesis to individual issues - particularly in explaining or analyzing the results of empirical studies of doctrinal issue outcomes in patent law - is incorrect. This is not to say that such studies cannot yield useful and insightful results, but that conclusions about their meaning should not be based on deviations from a fifty-percent success rate.