Art. 117(1) EPC does not contain an exhaustive list of admissible evidence, only mere examples. Parties to the proceedings are thus free in their choice of evidence. Any kind of evidence is admissible during proceedings before the EPO.

The EPC does not provide that certain questions of fact may only be proved by certain forms of evidence. Questions of fact must be settled on the basis of any credible information available (see e.g. J 11/88, OJ 1989, 433).

The admissibility of evidence should be clearly distinguished from the weight of evidence: whereas admissibility involves the question whether a piece of evidence should be considered at all, the probative value of evidence refers to the question whether the evidence to be considered provides sufficient proof of the alleged facts (see e.g. T 1698/08 on the distinction between admission of evidence and probative value of a document, T 1363/14 and T 838/92 about witness). The allegation that a witness might be biased does not itself render the testimony inadmissible; rather, suspicion of bias is a matter to be considered during the evaluation of evidence (T 838/92). See also IV.C.2.2.8 "Indication of facts, evidence and arguments – substantiation of grounds for opposition" and the case law reported there, for example T 234/86 (OJ 1989, 79), which stated that assessing the evidence is part of the process of ascertaining whether the opposition is well founded in substance (cf. T 353/06, T 1194/07). The principle of free evaluation of evidence applies only once evidence has been taken and cannot be used to justify not taking evidence offered. Moreover, no EPC provision requires that the facts adduced in support of an alleged prior use actually be proven within the opposition period in order to substantiate the allegation (T 1363/14, T 2238/15).

In T 1710/12, in the board's view, Art. 117(1) EPC gives no order of preference regarding the means of giving or obtaining evidence in proceedings before the EPO and it remains the free choice of a party to rely on the hearing of a witness (Art. 117(1)(d) EPC) EPC or on production of a sworn statement in writing (Art. 117(1)(g) EPC). For other findings that witness testimony do not necessarily carry less weight than documentary evidence, see also T 918/11 and T 2565/11, reported below, and T 441/04 (in which, however, the board first observed that documentary evidence was generally preferable to witness testimony for proving long-ago events); cf. however T 329/02.

The board in T 885/02 observed that the opinion of an expert does not necessarily reflect the view of the skilled reader for various reasons. Those experts who were in the case at issue eminent scientists had their own experience which was not necessarily common general knowledge. Those observations did not mean that those declarations were to be disregarded.