According to the established jurisprudence, a surprising effect (advantageous effect or feature) demonstrated in a comparative test can be taken as an indication of inventive step. If comparative tests are chosen to demonstrate an inventive step on the basis of an improved effect, the nature of the comparison with the closest state of the art must be such that the alleged advantage or effect is convincingly shown to have its origin in the distinguishing feature of the invention compared with the closest state of the art (T 197/86, OJ 1989, 371; T 234/03; T 378/03; T 568/11; T 1457/13; T 1521/13; T 1401/14) and alleged but unsupported advantages cannot be taken into consideration in respect of the determination of the problem underlying the invention (see also chapter I.D.4.2. "Alleged advantages"; T 20/81, OJ 1982, 217; T 561/94).

In T 197/86 (OJ 1989, 371) the board supplemented the principles laid down in earlier decision T 181/82 (OJ 1984, 401), according to which, where comparative tests were submitted as evidence of an unexpected effect, there had to be the closest possible structural approximation in a comparable type of use to the subject-matter claimed. In the case in point the respondent (patent proprietor) strengthened support for its claim by voluntarily providing comparisons with variants which, although not expressly belonging to the prior art, differed from the claimed subject-matter only by the distinguishing feature of the invention. The board summarised its position by stating, that in cases where comparative tests were chosen to demonstrate an inventive step with an improved effect over a claimed area, the nature of the comparison with the closest state of the art had to be such that the effect was convincingly shown to have its origin in the distinguishing feature of the invention. For this purpose it might be necessary to modify the elements of comparison so that they differed only by such a distinguishing feature (T 292/92, T 412/94, T 819/96, T 133/01, T 369/02, T 668/02, T 984/03, T 2043/09).

As early as T 35/85 the board had stated that an applicant or patentee may discharge his onus of proof by voluntarily submitting comparative tests with newly prepared variants of the closest state of the art identifying the features common with the invention, in order to have a variant lying closer to the invention so that the advantageous effect attributable to the distinguishing feature is thereby more clearly demonstrated (T 40/89, T 191/97, T 496/02).

It is for the applicant (patentee) to furnish evidence of an improved effect of the subject-matter of a claim, which has been asserted, but was not mentioned in the application as filed, in the whole of the claimed area vis-à-vis the closest prior art (T 355/97, T 1213/03, T 653/07).

In T 415/11 the board stated when the credibility that a technical effect is achieved by substantially all claimed compounds is at issue and in a situation where, it is prima facie unlikely that this is credible, it is not the opponent, but the patentee who has the burden of proving that the effect is achieved (see for example T 939/92, OJ 1996, 309; T 97/00).

In T 390/88 the board stated that where an alleged invention was prima facie obvious having regard to the prior art, it was however sometimes possible to prove inventiveness by comparative tests showing a significant improvement over the closest prior art. That situation was to be contrasted with other cases, in which it was not prima facie obvious to make the claimed compounds at all, and therefore comparative tests were not essential to establish inventiveness (see also T 656/91, T 930/99).

In T 702/99 the board stated that in cases concerning products such as cosmetics, in which applicants or patentees seek to establish that their inventions have an improved "feel" over the prior art, or opponents seek to deny such an improved "feel", it is common for one or more parties to file evidence of comparative tests conducted by a number of persons. It is essential for such tests to be made under conditions which ensure maximum objectivity on the part of those conducting the tests and who may be required at a later date to give evidence in proceedings. It is always desirable that such tests can be shown to be "blind" and that they have been conducted in the strictest conditions; that the testers have had no part in the making of the claimed invention or research leading up to the invention or the patenting procedure. See also T 479/06, T 275/11, T 1962/12, T 165/14, T 795/14, T 2304/16.

In T 234/03 the board stated that to be of relevance in demonstrating that a technical improvement is achieved in comparison with the closest state of the art, any comparative test presented must be reproducible on the basis of the information thus provided, thereby rendering the results of such tests directly verifiable (T 494/99). This requirement implies, in particular, that the procedure for performing the test relies on quantitative information enabling the person skilled in the art to reproduce it reliably and validly. Vague and imprecise operating instructions render the test inappropriate and thus irrelevant.

In T 172/90 the comparative examples produced did not constitute suitable evidence of inventive step. The board said that the products adduced as a basis of comparison were commercially available and had evidently been selected at random. Technical progress shown in comparison with products of this kind could not be a substitute for the demonstration of inventive step with regard to the closest prior art (see also T 164/83, OJ 1987, 149; T 730/96).