The burden of proof is determined by the legal cases which the respective parties are trying to make. Whether it is discharged or not is assessed by the board based on all the relevant evidence put before it. The burden of proof of insufficiency is as a general rule on the opponents, who should prove that despite making all reasonable efforts they were unable to put the invention into practice. If the patentee is claiming a result, which the prevailing technical opinion suggests is not achievable, and if the opponents are not able to repeat the method in the patent, they cannot be expected to do more than the patentee. Then, the burden of proof is on the patentee to show that the extraction method in the patent works as stated so that at least one way of putting the invention into practice has been given to the skilled person (approach set out e.g. in T 518/10, which in turn cited T 792/00 and T 1842/06).

In T 1608/13, referring to T 585/92, the respondent (opponent) argued that in appeal, after the opposition division had revoked the patent, the burden of proof shifted to the appellant to prove that the decision was wrong. However, the primary aim of appeal proceedings was to review the decision under appeal on the basis of the submissions and requests of the parties. This clearly encompassed the review of the opposition division's reasoning, in particular the assessment of the respondent's objections considered in the decision. If the reasoning as such was found to be wrong, there could not be any shift of the burden of proof on the substance. As regards the latter, it was established jurisprudence that a successful objection of lack of sufficiency presupposed that there were serious doubts substantiated by verifiable facts (T 967/09). It was primarily the respondent's duty to provide such facts in support of its objections.

T 30/15 (inter partes) contains extensive reasoning on the issue of burden of proof and on the submission of evidence in respect of Art. 83 EPC in conjunction with procedural aspects (RPBA 2007) especially at the appeal stage after a first instance decision revoking the patent for insufficiency of disclosure. The board found that, once the patent was revoked, it was up to the proprietor, as the appellant, to present a detailed line of argument in its statement of grounds of appeal, even if the grounds for the contested revocation seemed no longer to apply to a new set of claims. Thus, contrary to what was claimed by the proprietor, the respondents and former opponents were under no obligation to carry on proving the insufficiency of the disclosure of the invention if that was the reason for the patent's revocation. (Compare with T 1329/11, point 3.9 of the Reasons, where the appellant was the opponent but in a case where the application as filed did not provide a single example or other technical information from which it was plausible that the claimed invention could be carried out).

In T 1886/12, which also contains reasoning on the issue of burden of proof concerning alleged insufficiency of disclosure, the appellant (opponent) raised several objections but did not discharge the burden of proof.

In T 275/16 the board recalled that the burden of proof of insufficiency of disclosure is, as a general rule, on the opponent. However, in the case of an invention which goes against the prevailing technical opinion, it is the patent proprietor who needs to prove that the invention is sufficiently disclosed (T 792/00, points 3 to 5 of the Reasons; T 1842/06, point 3.4 of the Reasons; T 518/10, point 7.10.1 of the Reasons; cf. T 419/12, point 1.1.4(6) of the Reasons). In the case at issue, it was the prevailing technical opinion that no processes existed which would allow the production of titanium dioxide particles in a gas phase reaction, the particles all having the same particle diameter, i.e. having a monodisperse particle size distribution. Moreover, the patent did not contain data for the contentious Dtop/D50 value of 1. It was the patent proprietor (appellant) who had to show that it was possible to arrive at a Dtop (maximum particle diameter) / D50 (median particle diameter) ratio of 1, a value explicitly claimed, when using the process disclosed in the patent in suit. The board considered the requirements of sufficiency of disclosure not met, even with a skilled person construing the patent with a mind willing to understand (cf. T 190/99).

In T 2340/12, concerning the burden of proof, it was not contested that it is for the organ raising the objection of lack of sufficiency to justify its view. In ex-parte proceedings it is therefore up to the examining division or the board of appeal to substantiate the objection raised. Such objection should rely on concrete and verifiable knowledge or facts that question the reality of the effects provided for by the claimed invention. The lack of credibility could result, for example, from a conflict with established laws of physics. It was then for the applicant (appellant) to provide the arguments or evidence. The board remarked that the filing of experiments was not to be seen as an obligation imposed on the applicant but, in contrast, as a right, providing the applicant with the opportunity to convince that the examining division (or the board) had erred in its initial findings. In the case at issue, the invention related to fields of technology without any accepted theoretical or practical basis.

In T 2571/12 the board disagreed with the conclusions of the opposition division that, because no evidence had been provided by the opponent to show that any neuropsychiatric disorder could not effectively be treated using a glutathione precursor, the patent in suit was considered to disclose the invention in a manner sufficiently clear and complete for it to be carried out by a person skilled in the art. It was the patent that has to demonstrate the suitability of the claimed treatment for the claimed therapeutic indication.

It was decided in T 417/13 that physical and mathematical facts about particle size measurements were notorious. Therefore there was no need to discuss documents filed in order to establish the related facts.

See also chapter II.C.9. "Evidence"; T 63/06 (summary of its contribution in T 347/15), T 338/10 and T 967/09 in present chapter III.G.5.2.2.