Although the standard of proof is the same for all objections covered by Art. 100 EPC (cf. T 270/90, OJ 1993, 725), the case law identifies two levels or standards of proof to be applied with regard to disputes around public prior use - either the "balance of probabilities" or "up to the hilt". The standard of balance of probabilities is applicable when both the patent proprietor and the opponent had access to the material of which public prior use is alleged (see e.g. T 363/96, T 12/00, T 1105/00, T 2043/07). By contrast, in cases where all evidence in support of an alleged public prior use lies within the power and knowledge of the opponent, while the patentee has barely any or no access to it at all, it is incumbent upon the opponent to prove the alleged prior use up to the hilt (T 472/92, OJ 1998, 161; cf. T 782/92 referring to proof "beyond any reasonable doubt"). Subsequent case law also confirmed that, where public prior use is cited, the assessment of probability which normally underlies the boards' opinion must yield to a stricter criterion close to absolute conviction (T 97/94, OJ 1998, 467; see also T 848/94 and T 12/00). If any reasonable doubt exists as to what might or might not be the result of carrying out the literal disclosure and instructions of a prior art document, in other words if there remains a "grey area", then the case of anticipation based on such a document has to fail (T 793/93; see also T 464/94).
In T 2010/08, the board endorsed the opposition division's observation that the EPO subjected claims of prior use to a very strict standard, namely the (high) standard of proof beyond reasonable doubt. It did so, in particular, because virtually all the evidence stemmed from the opponent's knowledge, whereas the patent proprietor can only challenge that evidence by highlighting any inconsistencies or gaps. The board stated that the requirement to prove prior use beyond reasonable doubt could not be circumvented by overstretching the principle of ex officio investigation and so shifting the burden of proof from the opponent to the opposition division or the board of appeal. The board also took the view that, with regard to the complexity of prior uses, it was likewise relevant whether the opponent could have submitted evidence earlier, e.g. by offering a witness for examination, and whether it could have been expected to do so. In the case at hand, it held that it had not been established to the requisite level of certainty, i.e. beyond reasonable doubt, that the prior uses it had alleged had indeed occurred.
In T 743/89 it had been proved that a leaflet disclosing the invention had been printed seven months before the priority date, but it was uncertain when the leaflet had been distributed. Based on the balance of probabilities, the board concluded that the leaflet was available to the public before the priority date of the patent in suit and was, consequently, comprised in the state of the art. Although the date of distribution could no longer, i.e. 10 years on, be ascertained, it could reasonably be assumed that it had taken place within less than 7 months and had thus been completed well before the priority date of the patent in suit. The opposite assumption that the brochure had been kept confidential was not very plausible since it was in the patentee's own interest to ensure wide-spread distribution of the brochure in order to inform as many potential customers as possible of this latest development in a highly competitive field. Hence, the onus of proof was incumbent on the patentee who incidentally, being the originator of the pamphlet, should be in the possession of the necessary information.
In case T 1210/05, the board concluded that the contested prior disclosure via the public display of a poster identical to document (1) at an Edinburgh congress had not been sufficiently established, i.e. beyond all reasonable doubt, based on the following considerations: the fact that the various means of evidence, namely the submitted documents, the affidavits of the witnesses as well as their oral testimony, were not in did not contradict each other was not sufficient to meet the required standard of proof. Furthermore, the finding of the opposition division rested exclusively on the testimony of one of the witnesses. No independent evidence (in writing or by other persons) was available to support it. This did not mean that the written and oral testimony of the witness was per se insufficient. However, the board stated that there had to be good reasons for treating this evidence alone as having established the facts beyond any reasonable doubt.
In T 225/03 the opposition division had not taken evidence by hearing the witness before revoking the patent because it had considered the allegation of public prior use to be adequately proven by the documentary evidence. The board found that level of proof to be insufficient, and held that the respondent's request – that the documentary evidence be complemented by hearing the witness – had to be granted, as only the witness could confirm the links between the individual pieces of evidence and the circumstances of use as explained in his statutory declaration. It therefore sent the case back to the department of first instance for further prosecution.
In T 441/04 the board agreed with the respondent that evidence in document form was normally preferable to witness testimony about long-ago events. But that did not mean that in the case in point such testimony was necessarily excluded per se or less convincing than documentary evidence. Applying the (high) standard of proof beyond doubt, the board considered that it had taken due account of the fact that the alleged public prior use – proven by witness testimony – was attributable to the appellant. The evidence submitted (mainly witness testimony) had measured up to that standard, and therefore no new or additional proof (such as drawings) was needed.
In T 1914/08, the board – like the opposition division at first instance – regarded two witnesses' testimonies as the decisive evidence proving without any gaps the alleged prior public use to the standard required in the case at hand. Each testimony, considered in isolation, gave a consistent and full picture of the process in question. Accordingly, it endorsed the opposition division's evaluation of the evidence, the standard of proof applied by it ("beyond all reasonable doubt") and the result of its analysis. It held, moreover, that the patent proprietor/appellant's request for additional evidence had to be refused. Since the opposition division had, in the course of its unfettered consideration of the evidence, concluded that the alleged prior public use had, essentially, been established by the two witnesses' testimonies and since the appellant had been unable to cast doubt on the probative value of those testimonies, there was no need for the opponent/respondent to adduce further evidence in either the opposition or the appeal proceedings (see also Chapter IV.C.1.4.6).