In decision T 94/82 (OJ 1984, 75) the board ruled that the requirement of clarity could be fulfilled in a claim to a product if the characteristics of the product were specified by parameters related to the physical structure of the product, provided that those parameters could be clearly and reliably determined by objective procedures which were usual in the art (see also T 452/91, T 541/97, T 437/98, T 193/01). It is not mandatory to give instructions in the claim itself as to how the product is to be obtained (T 94/82).

In T 849/11 the board summarised case law on the requirements of Art. 84 EPC with regard to the characterisation of an invention by parameters. The board stated that: i. the claims must be clear in themselves when read by the skilled person; and ii. the method for measuring a parameter (or at least a reference thereto) must appear completely in the claim itself (see T 1156/01, T 412/02, T 908/04, T 555/05, T 1497/08), and that iii. the applicant who chooses to define the scope of the claim by parameters should ensure that a skilled person can easily and unambiguously verify whether he is working inside or outside the scope of the claim. The board further stated that the requirements of Art. 84 EPC can still be met if it can be convincingly shown that: i. the method to be employed belongs to the skilled person's common general knowledge, or ii. all the methodologies known in the relevant technical field for determining this parameter yield the same result within the appropriate limit of measurement accuracy (see T 1156/01).

In T 29/05 it was found that although different experimental protocols might be applied in the case in point for assessing hybridisation under stringent conditions, they were usual in the art (following T 1084/00).

In T 307/06 the claims contained the added feature "and a T_g of less than 25°C". The board noted that there were different methods for determining T_g (glass transition temperature). However, the mere fact that several methods existed did not render the claims unclear if at least one of the following conditions was met: (a) the different methods yield essentially the same T_g values for the same material, or (b) the person skilled in the art had associated the range of T_g values mentioned in claim 1 at issue with only one standard method of measurement. Neither condition (a) nor (b) was fulfilled. The board concluded that this left doubt as to which subject-matter was covered by claim 1, and thus rendered said claim unclear (see T 728/98, OJ 2001, 319; T 306/13).

In a number of decisions the boards stress that it must be clear to the skilled reader from the claim itself how the parameters are to be determined, unless it can be demonstrated that the skilled reader would immediately know which method and conditions to apply even without any such indication in the claim.

In T 412/02 the board held that the unambiguous characterisation in a claim of a product by parameters or by a mathematical relation between parameters necessarily required that each parameter could be clearly and reliably determined. In the board's view, it followed that the knowledge of the method and conditions of determination of the parameter was necessary for the unambiguous definition of the parameters and, as a consequence, for the unambiguous definition of a mathematical relation between them. Thus, in order to allow the matter for which protection was sought to be defined, it had to be clear from the claim itself when being read by the person skilled in the art exactly how the parameters should be determined. This would, as a rule, imply that the method of determination and the conditions of measurement which might have an influence on the value of the parameter should be indicated in the claim, either expressly or, if appropriate, by way of reference to the description according to R. 29(6) EPC 1973. Such indication would only become superfluous provided it could be shown that the skilled person would know from the outset which method and conditions to employ. See also T 1156/01.

In T 1819/07 the board held that the parameter "average particle size" rendered the claim unclear as neither the type of average (volume, surface, number) nor a method for determining it was indicated in the claims. See also T 967/08 and T 45/10.

In T 992/02 the board took the view that whilst noting the general principle that the claims must be clear per se, the non-inclusion in the claim of the method whereby one of the parameters characterising the composition claimed (level of formation of volatiles) could be measured was justified by the requirement that the claims be concise. It considered that such a conclusion could be reached in the case in hand because the method was clearly identified in the description and did not give rise to any ambiguity.

In T 2086/11 the parameter "mean aspect ratio" represented the mean of the individual aspect ratios, defined as the ratio of the major axis to thickness, of 50 aluminium flake particles having major axes of not more than 10 μm. The board held that this parameter did not enable to distinguish which subject-matter is covered by the claims and which is not. According to the method of measuring explained in the description, the investigated sample contained thousands of particles having a major axis of not more than 10 μm and the method described in the application did not give any precise indication of how to select such 50 particles. One operator, by selecting at random one group of 50 particles, could measure a mean aspect ratio within the claim whilst another operator, by selecting a different group of 50 particles, could find a value of mean aspect ratio outside the claim. The parameter "mean aspect ratio" was thus by itself insufficient to characterise the claimed product in a reliable way and did not comply with Art. 84 EPC 1973.