2.2. Distinction between the admissibility of evidence and its probative value

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 with regard to a document, T 1363/14 and T 838/92 with regard to witnesses).

The principle of free evaluation of evidence applies only once evidence has been taken and cannot be used to justify not taking evidence offered (T 2238/15) – confirmed by G 2/21, points 44 and 56 of the Reasons (with reference to T 1363/14 and T 2238/15).

Assessing the evidence is part of the process of ascertaining whether the opposition is well founded in substance (T 234/86; along the same lines T 353/06, T 1194/07). No EPC provision requires that an alleged prior use must actually be proven within the opposition period in order to substantiate the allegation (T 1363/14, T 2238/15).

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).

In T 104/23 the appellant (patent proprietor) submitted that the opponent had spoken with the witness just before the witness was heard, ignoring the instructions of the opposition division to refrain from doing so. The board was not aware of any procedural rule that would require a division or a board to reject evidence out of hand because instructions were not properly complied with. Neither did there seem to exist a basis for a bar to using a witness statement under these circumstances. The correct approach in these circumstances appeared to the board to be to consider the witness's disregard of the division's instructions in the evaluation of evidence, namely the witness's credibility. Contact between the opponent and witness during an interruption of oral proceedings might potentially raise doubts about the witness's impartiality and therefore diminish the probative value of their testimony. However, these were merely factors to consider when assessing the witness's credibility, especially considering the – undisputed – link between the witness and the opponent.

In T 2892/19 the appellant (patent proprietor) alleged that K was an unreliable witness because they had an interest in the invalidation of the patent and was involved in criminal proceedings. The board considered that the considerations about K's status and interests had to be considered in the evaluation of the submitted evidence; such considerations did not render evidence or witness statements automatically inadmissible or unreliable in proceedings before the EPO. Rather, it is not exceptional in opposition proceedings that a piece of evidence or a witness statement stems from an interested person or even a party to the proceedings.

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.

In T 1551/14 after the first oral proceedings the patent proprietor had filed a new auxiliary request in which the subject-matter of the two independent claims had been limited by a new feature. After the summons to a second round of oral proceedings, and within the period fixed under R. 116 EPC, the opponent had filed a statutory declaration from a witness who had already been heard and that this witness be heard again. Among other things, the board held that not even contradictions with previous submissions – which it did see in the case in hand – could justify refusing from the outset to admit submissions that were a legitimate reaction to a change in the counterparty's case.