Patent cases use a "preponderance of the evidence" standard of proof, unless the validity of a patent is in question, in which case the standard is "clear and convincing evidence." The comment examines the meaning of the standards in their application to United States patent practice. After giving examples with interference and infringement cases, the comment argues that the vast majority of cases are decided on evidence not presented to the Patent and Trademark Office (PTO) during prosecution of the patent. The assumption that a patent is valid is based on the thorough examination it receives prior to its allowance by the PTO. But if cases are decided on evidence not available to the PTO or not used by the PTO, there is no reason for a presumption of validity. Therefore, the comment argues, only a "preponderance of the evidence" standard should apply in patent litigation.
David W. Okey, Issued Patents and the Standard of Proof: Evidence Clear and Convincing or Merely Ponderous, 17 J. Marshall J. Computer & Info. L. 557 (1999)