Book excerpt: Before 1982, rebutting a patent's presumption of validity generally required clear and convincing evidence. The standard would shift to a preponderance of the evidence, however, if a validity challenger introduced “unconsidered evidence,” that is, prior art not considered during examination by the United States Patent and Trademark Office (“PTO”). Since 1982, rebutting a patent's presumption of validity has required clear and convincing evidence in all instances. The authors examined and compared empirical data from the pre- and post-1982 periods to determine the extent to which jury verdicts were affected by unconsidered evidence, and the extent to which these verdicts may have been affected by the corresponding evidentiary standards applied by the courts. During the pre-1982 period, validity challengers introducing unconsidered evidence succeeded in establishing the factual bases of invalidity to juries more often than challengers relying on only considered evidence - thirty-two percent to thirteen percent, respectively. Two factors may have contributed to this result: 1) the relative evidentiary impact of unconsidered evidence over considered evidence, and 2) the reduced difficulty of satisfying a preponderance of the evidence standard instead of a clear and convincing evidentiary standard. The degree to which these factors individually contributed to a validity challenger's likelihood of success could not be determined from the pre-1982 data because the two factors did not appear separately during this period; shifting the standard depended on the challenger introducing unconsidered evidence. Comparing the pre-1982 data with the post-1982 data permits the two factors to be decoupled, because, during the post-1982 period, challengers introducing unconsidered evidence uniformly operated under a clear and convincing evidentiary standard. During the post-1982 period, validity challengers established the factual bases of invalidity thirty-four percent of the time. Thus, in both periods, challengers introducing unconsidered evidence succeeded approximately one third of the time, even though the evidentiary standards were different during the two periods. This observation suggests that a challenger's likelihood of establishing the factual bases of invalidity is driven more by the evidentiary impact of unconsidered evidence than a shift to a preponderance of the evidence standard.