According to established case law, a central consideration in selecting the closest prior art is that it must be directed to the same purpose or effect as the invention, otherwise it cannot lead the skilled person in an obvious way to the claimed invention.
According to T 606/89 the closest prior art for the purpose of objectively assessing inventive step is generally that which corresponds to a similar use requiring the minimum of structural and functional modifications (see also T 574/88, T 606/89, T 686/91, T 834/91, T 482/92, T 59/96, T 650/01, T 1747/12). In T 273/92 the board confirmed the established case law that a document could not qualify as the closest prior art to an invention merely because of similarity in the composition of the products; its suitability for the desired use of the invention also had to be described (see also T 327/92). According to T 506/95 the closest prior art was the art most suitable for the purpose claimed by the invention, not the art superficially showing structural similarities. Ideally, that purpose should already be mentioned in the prior art document as a goal worth achieving (T 298/93, T 859/03). The aim was that the assessment should start from a situation as close as possible to that encountered by the inventor. The real-world circumstances had to be taken into account. If it was not clear what the closest prior art was, the problem and solution approach should be repeated taking possible alternative starting points (T 710/97, T 903/04, T 2123/14).
In T 2255/10 (citing T 482/92), the board stated that in establishing the closest prior art, the determination of the purpose of the invention is not to be made on the basis of a subjective selection from among statements of purpose which may be set out in the description of the application, without any reference to the invention as defined in the claims. On the contrary, the question to be asked is, what, in the light of the application as a whole, would be achieved by the invention as claimed. For this reason, statements of purpose must be read in conjunction with the claims. Merely inserting such a statement into the description does not entitle an applicant effectively to "veto" any inventive step objection based on a document which is unrelated to this purpose, if it is not plausible that the invention as claimed would actually achieve the stated purpose.
In T 53/08 the board found that the disclosure in each of the two documents (1) and (10) differed from the patent in suit in respect of just a single feature. In deciding whether document (1) or document (10) had to be regarded as the closest prior art, it considered the patent's objective, which was to develop the highly effective herbicidal ingredient of formula (A1) in such a way that it did not significantly damage crops when used in a concentration with herbicidal effect. The natural starting point for the invention was therefore the document disclosing the active ingredient of formula (A1).
In T 2571/12 the board held that, in the case of claims directed to medical uses, the closest prior art is usually a document disclosing the same therapeutical indication.