Neither in the EPC nor in the case-law of the board of appeal are there formal rules laid down for the evaluation of evidence. Thus the EPO departments have the power to assess whether the alleged facts are sufficiently established on a case-by-case basis. Under the principle of unfettered consideration of evidence, the respective body takes its decision on the basis of all of the evidence available in the proceedings, and in the light of its conviction arrived at freely on the evaluation whether an alleged fact has occurred or not (see e.g. T 482/89, OJ 1992, 646; T 592/98, T 972/02). However, the principle of free evaluation of evidence applicable to the proceedings before the EPO cannot go as far as to justify the refusal of a relevant and appropriate offer of evidence. Free evaluation of evidence means that there are no firm rules according to which certain types of evidence are, or are not, convincing. It does not mean that the deciding body can choose the evidence which it considers sufficient for establishing the truth, but that there are no firm rules according to which certain types of evidence are, or are not, convincing (T 474/04). On the other hand, failure to submit evidence despite a board's request to do so may be viewed as a sign that the evidence would perhaps not confirm what has been claimed (see T 428/98).
When evaluating evidence, it is necessary to distinguish between a document which is alleged to be part of the state of the art within the meaning of Art. 54(2) EPC, in the sense that the document itself is alleged to represent an instance of what has been made available to the public before the priority date of the opposed patent, and a document which is not itself part of the state of the art, but which is submitted as evidence of the state of the art or in substantiation of any other allegation of fact relevant to issues of novelty and inventive step (T 1110/03, OJ 2005, 302). In the first situation, a document is direct evidence of the state of the art; its status as state of the art cannot normally be challenged except on authenticity. In the second situation, a document is also evidence albeit indirect; it provides a basis for an inference about, e.g. the state of the art, common general knowledge in the art, issues of interpretation or technical prejudice etc. – an inference which is subject to challenge as to its plausibility. Only a document of the first kind can be disregarded on the sole ground that it is published after the priority date. Documents of the second kind do not stand or fall by their publication date even on issues of novelty and inventive step. Disregarding indirect evidence would deprive the party of a basic legal procedural right generally recognised in the contracting states and enshrined in Art. 117(1) and Art. 113(1) EPC.
Following the above distinction, the board in T 1797/09 held the comparative examples in document D1 that had not been made available to the public at the priority date, i.e. a document that qualified as indirect evidence, to be equivalent to the comparative examples put forward by a party as proof of its allegations. Such examples were allowable at any reasonable point of the proceedings, provided they were not in abuse of the proceedings. In the case at issue, the comparative examples in document D1 cast doubt on the allegation that the claimed features would solve the technical problem in a non-obvious manner. The patent was thus revoked for lack of inventive step.