According to the case law of the boards of appeal (see T 119/82, OJ 1984, 217; T 48/86), inventiveness can sometimes be established by demonstrating that a known prejudice, i.e. a widely held but incorrect opinion of a technical fact, needs to be overcome. In such cases, the burden is on the patentee (or patent applicant) to demonstrate, for example by reference to suitable technical literature, that the alleged prejudice really existed (T 60/82, T 631/89, T 695/90, T 1212/01). A high standard of proof to demonstrate prejudice is required (T 1989/08).

A prejudice in any particular field relates to an opinion or preconceived idea widely or universally held by experts in that field. The existence of such prejudice is normally demonstrated by reference to the literature or to encyclopaedias published before the priority date. The prejudice must have existed at the priority date, any prejudice which might have developed later is of no concern in the assessment of inventive step (T 341/94, T 531/95, T 452/96, T 25/09).

Generally speaking, prejudice cannot be demonstrated by a statement in a single patent specification, since the technical information in a patent specification or a scientific article might be based on special premises or on the personal view of the author. However, this principle does not apply to explanations in a standard work or textbook representing common expert knowledge in the field concerned (T 19/81, OJ 1982, 51; T 104/83; T 321/87; T 392/88; T 519/89; T 453/92; T 900/95; T 1212/01). In T 515/91 the board regarded "ABC Naturwissenschaft und Technik" as a standard work (see also T 461/92, T 152/93). In T 943/92 the existence of a prejudice was supported by a specialist book which reflected the technical knowledge in the special field of the contested patent. This book did not contain the opinion of just one specialist author, but that of experts in the field, as it had resulted from the collaboration of "numerous recognised scientists, technicians and practicians as well as associations and institutes". General critical remarks in one textbook were not sufficient to substantiate an alleged prejudice, if a plurality of prior art documents pointed to the opposite (T 134/93).

Generally speaking, established board of appeal case law is very strict when it comes to recognising the existence of a prejudice. A solution put forward as overcoming a prejudice must clash with the prevailing teaching of experts in the field, i.e. their unanimous experience and notions, rather than merely cite its rejection by individual specialists or firms (T 62/82, T 410/87, T 500/88, T 74/90, T 943/92, T 531/95, T 793/97, T 2453/09). A declaration by a technical expert in the field was not interpreted as evidence that there was a prejudice in the prior art at the priority date; rather, this declaration was taken as the opinion of one expert, issued almost ten years after the priority date (T 2044/09). The fact that a disadvantage is accepted or the prejudice simply ignored does not mean that a prejudice has been overcome (T 69/83, OJ 1984, 357; T 262/87; T 862/91).

The board in T 1989/08 observed that this meant the standard of proof was almost as a high as that required for common general knowledge in the art. For example, it is not enough that the opinion or idea is held by a limited number of individuals or that it is a prevalent view within a given firm, however large. In the case in hand all the evidence offered in support of the alleged prejudice amounted to no more than ten documents, all of which were either specialist papers or patents. This small number of publications intended for a select readership in the field was in itself a tenuous basis for asserting prejudice (see also T 25/09).

In T 1212/01 the patent related to pyrazolopyrimidinones for the treatment of impotence (Viagra). The patentee referred to some thirty scientific articles in order to demonstrate the existence of a technical prejudice that drugs lowering blood pressure were a cause of impotence rather than a form of treatment for that condition. The board, however, stated that the contents of such a selection from the prior art could not be considered per se as creating a technical prejudice against oral treatment of male erectile dysfunction. Such a prejudice could only be established by proving that, in relation to the technical solution, a relatively widespread error or misapprehension about the technical invention existed among skilled workers in the relevant field before the priority date of the patent in suit. This was not the situation in the case in hand.

In T 550/97 the respondent (patent proprietor) had argued that, years after the invention, technically less advanced solutions had been filed for and marketed as a means of integrating different mobile radio networks. However, the board did not consider the fact that arguably less advanced solutions had been developed later, to be evidence of inventive step, as there was no reason to assume that later development was attributable to a technical prejudice which the invention had needed to overcome.

In T 347/92 the board pointed out that the finding of a relatively small operating window in an area which, according to the teaching of the most recent publications, was considered inaccessible, could not be considered obvious to a person skilled in the art.

One form of secondary indicia in the nature of a "technical prejudice" is a development of the art in a different direction (T 24/81, OJ 1983, 133; T 650/90; T 330/92).

In T 883/03 the board found that the teaching that could have led to the characterising feature of claim 1 had long formed part of the state of the art; yet for all that time experts had been "blind" to that knowledge. In the case in point that was a further indication of the inventiveness of the solution proposed in claim 1.

In T 872/98 the board pointed out that the presence of secondary indicia might also be attested by the fact that a competitor had, shortly after the priority date, filed a patent application with the German Patent Office in which the invention took an entirely different direction to the European application.

In T 779/02 the board indicated that a prejudice could be proved by the fact that the closest prior art and the invention were separated by a long period of time (over 16 years in the case in hand), during which time the only solutions pursued led away from the invention, and the solution provided by the invention only becoming acceptable to experts in the field after this time.