When evidence is submitted, the first thing to establish is what fact is being asserted, and then whether that fact is relevant to the decision. If not, the assertion is no longer considered and the evidence is not examined further. If the alleged fact is relevant, the next point is whether it is proven by the evidence submitted.
When evidence is examined, since the EPC says nothing about how the outcome of taking of evidence should be assessed, the principle of unfettered consideration applies. This means that its content and its significance for the proceedings are assessed in the light of the particular circumstances of each individual case (e.g. time, place, type of evidence, position of witness in firm, etc.). The principle of unfettered consideration also means that EPO departments are empowered to evaluate evidence submitted by the parties in any appropriate manner, or indeed to disregard it as unimportant or irrelevant. In particular it has to be decided on a case-by-case basis when a particular piece of evidence is sufficient.
When deciding whether an alleged fact is accepted, the division may use the criterion of the "balance of probabilities", which means that it is satisfied that one set of facts is more likely to be true than the other. Furthermore, the more serious the issue, the more convincing must be the evidence to support it (see T 750/94). For example, if a decision might result in revocation of the patent in a case concerning alleged prior use, the available evidence has to be very critically and strictly examined. In particular, in case of alleged prior use for which little if any evidence would be available to the patentee to establish that no prior use had taken place, the division should cede to the stricter criterion close to absolute conviction, i.e. beyond any reasonable doubt (see T 97/94).
When parties make conflicting assertions, the division must decide which evidence is the most convincing. If it cannot establish which allegation is right on the basis of the evidence put forward, it must decide on the basis of the burden of proof, i.e. against the party bearing that burden but unable to prove its point convincingly.