The EPO departments decide on the issues that arise before them on the basis of the evidence adduced by the parties. Their decisions need not, and in most cases cannot, be based on absolute conviction, but instead are to be arrived at on the basis of the overall balance of probabilities, in other words on the footing that one set of facts is more likely to be true than the other. This standard applies particularly in opposition appeal proceedings where the boards of appeal are called upon to reach a conclusion on the basis of the overall balance of probabilities, as distinct from "beyond all reasonable doubt" or "absolute conviction". Each of the parties must therefore seek to prove facts alleged by it to that degree of proof (see e.g. T 182/89, OJ 1991, 391; T 270/90, OJ 1993, 725; T 859/90, T 109/91, T 409/91, OJ 1994, 653; T 1054/92, T 296/93, OJ 1995, 627; T 326/93, T 343/95, T 363/96). The standard of balance of probabilities applies also in decisions issued in ex parte proceedings (T 381/87, OJ 1990, 213; T 69/86, T 128/87, OJ 1989, 406; T 939/92, OJ 1996, 309).
When an issue of fact is being examined and decided by the EPO on the balance of probabilities, the more serious the issue the more convincing must the evidence be to support it. If a decision on such an issue may result in refusal or revocation of a European patent, for example, in a case concerning alleged prior publication or prior use, the available evidence in relation to that issue must be very critically and strictly examined. A European patent should not be refused or revoked unless the grounds for refusal or revocation (that is, the legal and factual reasons) are fully and properly proved (T 750/94, OJ 1998, 32).
In some cases, the boards require a "sufficient degree of certainty" before deciding on the case based on the evidence available (see e.g. T 100/97, T 1842/06). In accordance with the established jurisprudence of the boards of appeal, in the case of a proposed amendment under Art. 123(2) EPC or of a correction under R. 139 EPC, the factual disclosure of a European patent application as originally filed has to be established to a rigorous standard, namely the standard of certainty "beyond reasonable doubt". In T 1248/08 the board could not follow the arguments of the appellant adopting proof "on the balance of probability" by putting forward the results of a survey conducted amongst partners and staff at the firm of its representative. A question of accuracy and disclosure could not be decided by a poll.
In cases concerning an application for re-establishment of rights in respect of the time limit for filing the statement of grounds according to R. 136 EPC (former Art. 122(2) and (3) EPC 1973), the application is regarded as duly supported only if it is clear from the facts set out and credibly substantiated ("glaubhaft gemacht") that the applicant took all due care required by the circumstances to observe the time limit (see T 13/82, OJ 1983, 411; cf. also T 243/86). It is not further specified whether this standard involves a lower degree of probability, as the term "Glaubhaftmachung" for the purposes of German, Austrian and Swiss law.