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. If the result of the boards' evaluation of the evidence is such as to persuade them one way or another, then the balance of probabilities standard is met. 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" (on the latter concept, refer to the more recent case T 2451/13). 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 of 20 June 1996 date: 1996-06-20; 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 date: 1988-06-03, OJ 1989, 406; T 939/92, OJ 1996, 309; T 545/08 on internet publications).

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; T 329/02; T 750/94 was cited and considered in the context of evidence by the board in T 545/08). See Guidelines G‑IV, 1 – November 2015 version): "If the applicant shows sound reasons for doubting whether the document forms part of the 'state of the art' in relation to his application and any further investigation does not produce evidence sufficient to remove that doubt, the examiner should not pursue the matter further." See also the version currently in force, G‑IV, 1 – November 2018 version.

The board in T 286/10, citing T 472/92, observed that it is settled case law that evidence is generally assessed in terms of what seems most probable; it made an exception only for public prior use objections where practically all the supporting evidence lay within the power and knowledge of the opponent (see in this chapter III.G.4.3.2 "Public prior use"). It considered the balance of probabilities standard to be met if, after evaluating the evidence, a board was persuaded one way or the other.

For internet publications, the standard of proof is the balance of probabilities (T 286/10, T 2227/11, T 1711/11, T 353/14, T 545/08. See also chapters I.C.3.2.3 "Internet disclosures" and I.C.3.5.2 c) "Internet – proof of the date of availability" with reference to the Guidelines and OJ 2009, 456-462).

In the board's judgment in T 658/04, an expert's declaration which is not supported by verifiable facts but which merely constructs some hypotheses, cannot reflect the common general knowledge to be considered for assessing the sufficiency of disclosure within the meaning of Art. 83 EPC. For this reason, document (12) – declaration of U.K Pandit submitted by the appellant (patent proprietor) as an expert opinion – did not form part of the common general knowledge. T 658/04 also summarised what is part of the common general knowledge according to the case law of the boards of appeal.

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 (see chapter III.E.4.4.).