The boards of appeal have developed in their case law certain principles as to the standard of proof required to establish the facts on which a decision is to be based. In some decisions the boards of appeal have applied the standard of "the balance of probabilities", which means that in relation to, for example, the question of when a document was first made available to the public, the board must decide what is more likely than not to have happened (see for example decisions T 381/87, OJ 1990, 213; T 296/93, OJ 1995, 627; and T 729/91 of 21 November 1994). In other decisions the boards have taken the view that a fact has to be proved "beyond reasonable doubt" or "up to the hilt" (see e.g. T 782/92 of 22 June 1994; T 97/94, OJ 1998, 467; T 848/94 of 3 June 1997; T 472/92, OJ 1998, 161; and, in particular, T 750/94, OJ 1998, 32)(see also T 313/05, T 1335/05).
As far the standard of proof to be applied is concerned, the boards' established legal practice is to apply the same standard of proof to prior public use objections as to other objections under Art. 100 EPC: the balance of probability (T 472/92, OJ 1998, 161). In T 270/90 (OJ 1993, 725) the board expressly refused to accept a submission that the much more rigorous standard of "beyond all reasonable doubt" should apply, even if, as was the case, both parties were able to access and adduce evidence relating to the decisive issue of confidentiality of the transactions alleged to constitute prior public use. It stated that, in opposition proceedings, decisions of the boards on the evidence adduced by the parties had to be arrived at on the basis of the overall balance of probability, as distinct from "beyond all reasonable doubt" or "absolute conviction". Each of the parties therefore had to seek to prove facts alleged by it to that degree of proof.
In T 472/92 (OJ 1998, 161) the board noted, however, that although the standard of proof was the same for all objections covered by Art. 100 EPC 1973, in those prior public use cases where practically all the evidence in support of an alleged prior public use lay within the power and knowledge of the opponent, the latter had to prove his case up to the hilt. The board noted that, generally speaking, in cases not involving the issue of prior public use, both parties were able to obtain and adduce the evidence upon which their respective cases were based. By contrast, in the majority of prior public use cases, practically all the evidence in support of an alleged prior public use lay within the power and knowledge of the opponent, so the patentee seldom had any ready, or indeed any access to it at all. As a consequence, an opponent had to prove his case up to the hilt, for little if any evidence would be available to the patentee to establish the contradictory proposition that no prior public use had taken place (T 97/94, OJ 1998, 467).
In T 665/00, the board held that the evidence submitted had to be assessed in accordance with the principle of "unfettered consideration of the evidence". Moreover, the same principles of taking evidence applied to all the facts and arguments relied on in support of all the grounds of opposition, including public prior use (see, to that effect, T 270/90, OJ 1993, 725). However, the board pointed out that the exercise of this unfettered discretion meant that varying standards of strictness could be applied in evaluating the evidence. Thus, where an issue of fact was being decided on the balance of probabilities, the more serious the issue and its consequences for the fate of the patent, the more convincing the evidence had to be (T 750/94, OJ 1998, 32). In particular, if the board's decision on whether to revoke the patent depended on that issue of fact, the available evidence would have to be examined very critically and strictly.