The probative value of the declarations of a witness depends on the circumstances of the particular case (T 937/93, cited in T 190/05). The credibility of witnesses cannot be impugned merely because they are related to one another and have a business relationship with one of the parties (T 363/90). An allegation based solely on suspicion cannot reasonably be expected to form a valid ground for casting doubt on the credibility of the evidence (see T 970/93 involving allegations of falsified evidence). The statements of employees of one of the parties were regarded as sufficient evidence in a series of appeal cases, e.g. T 162/87 and T 627/88, T 124/88, T 482/89 (OJ 1992, 646), T 363/90, T 830/90 (OJ 1994, 713), T 838/92 and T 327/91, T 190/05, J 10/04 (employee of representative's law firm).
In J 10/04, the Receiving Section had doubted the credibility of sworn statements given by a legal assistant on the basis that they were not persuasive because she was not impartial and her statements were based on personal impressions. It had not invited her to be heard in person as a witness because it had considered that this would not lead to a different its evaluation of the evidence. The Legal Board held, first of all, that a sworn statement was a form of evidence with a high probative value, especially if it was given, as in the case in point, in the knowledge that wilfully false statements were punishable under the applicable law. Strong reasons were therefore needed to disregard this kind of evidence, for example a set of circumstances making the statement very unlikely. If the Receiving Section had considered the circumstances described in the statement very unlikely, it ought to have heard the witness personally in order to evaluate her credibility (the ruling in J 10/04 has been applied in e.g. T 1100/07, in which the board reiterated some of the rules on hearing witnesses).
With regard to the credibility of witnesses, the board observed in T 1210/05 that even a person who was not being dishonest might make untruthful statements. A person can be honestly mistaken in his recollection of an event, particularly if the event took place some time ago.
In T 100/97, without casting doubt on the good faith of the submitted declaration, the board stated that for the content of such statements to be considered sufficiently credible, it had to be corroborated by documents of a definite date. In the absence of any such documents, the board decided that it could not be established with a sufficient degree of certainty what had been made available to the public before the priority date of the opposed patent.
In T 804/92 (OJ 1994, 862) the opposition division had, in a communication to the parties, suggested in detail the content of a statement under oath. Such a practice was firmly rejected by the board because it involved the risk of leading witnesses and could seriously undermine the probative value of such statements. This applied to departments at any instance in proceedings before the EPO.
In T 1191/97 the appellant's criticism of the evaluation of evidence by the department of first instance did not pose a serious threat to the witness's credibility. The fact that the events at issue had taken place a long time ago could readily explain certain imprecisions in the witness's testimony. The board saw no indication that the witness had been in breach of his obligation to testify to the best of his recollection.
In T 61/07 the board made clear that the doubts cast by the respondent on the witness's memory about events which had happened 23 years ago concerned not the witness's credibility but the credibility of the testimony. However, it saw no reason to doubt that his testimony was credible. Just because the witnesses had each independently met a third witness before being heard did not automatically mean that their recollections had been influenced. Shortly before a party alleges prior use, witnesses are normally sounded out about what they actually remember. Such a discussion with a potential witness did not automatically imply that during it the party or one of its staff had influenced what the party remembered.
In T 905/94, the board held that the fact that one witness had made his declaration three years earlier than other witnesses was not sufficient reason to make his testimony more credible.
In T 473/93 the board decided that the appellant's surmise that he had been in error in making his statutory declaration was not sufficient to allow orally presented facts which deviated from the declaration to appear more credible. The fact alleged orally therefore had to be regarded as not proven.
In T 361/00, the appellant (opponent), seeking to establish novelty-destroying public prior use during the opposition proceedings, had submitted a technical report and a statutory declaration ("eidesstattliche Erklärung") relating to visits to a cement works and proposed that the declaration's author additionally be heard as a witness. On appeal, it submitted two new documents, one being a second statutory declaration by the same author. The board went into the circumstances of the visits and the particular features of cement works in some detail and, observing that it was common ground between the parties that the technical report had not been made available to visitors, held that the fact that they could have found out what was in it on request did not mean its content had been made publicly available. Visiting a cement works was not the same as visiting a library. As to the two statutory declarations, the board had no doubt that they had been made in good faith but found them to be contradictory. Having found that the appellant's submissions at the oral proceedings had not clarified the contradictions, it concluded that the appellant had failed to furnish conclusive proof of the alleged prior use. Lastly, it held that there was no need to hear the witness, giving reasons for this finding (on contradictory statements, see also T 833/99).
In national proceedings before the Dutch District Court and Court of Appeal, several witnesses had been heard, and written statements had been introduced by both parties. The Dutch Court of Appeal could not establish an uninterrupted chain of proof that the alleged public prior uses had indeed taken place before the priority date of the contested patent. After evaluating the Dutch Appeal Court's findings, the board's judgment in T 665/95 was that no further investigations were necessary and that the conclusion of the Dutch Appeal Court could be followed, with the consequence that the subject‑matter of the granted claim 1, contrary to the impugned decision of the opposition division, was considered to be novel. (Other examples of cases in which findings made in national proceedings served as evidence include T 760/89, OJ 1994, 797 (USA); T 582/90 (FR), T 1043/93 (IT), T 885/02 (NL); T 276/07 (IT); T 407/08 (DE); R 21/09 (UK); T 1904/12 (DE) and T 202/13 (NO)).
An unsigned statement by an unknown and unnamed person should in principle be given minimal weight (T 750/94, OJ 1998, 32). In T 212/97, the board could not regard the citation of facts based on hearsay and not accompanied by a witness testimony as adequate evidence for the alleged prior uses.