Where the person skilled in the art has to find out by trial and error which, if any, compound meets the parameter set out in the claim, this constitutes an undue burden. The fact that this could be done by routine experimentation was not sufficient for the subject-matter claimed to meet the requirements of Art. 83 EPC. Nor did the question whether or not the parameter could be reliably determined play a role (T 339/05). In T 123/06 the board found that the functional definition of the device was no more than an invitation to perform a research programme, the skilled person only being able to establish through trial and error whether the claimed device was achieved. This amounted to an undue burden.
According to T 1063/06 (OJ 2009, 516), a functional definition of a chemical compound (in this case in a reach-through claim) covered all compounds possessing the capability according to the claim. In the absence of any selection rule in the application in suit, the skilled person, without the possibility of having recourse to his common general knowledge, had to resort to trial-and-error experimentation on arbitrarily selected chemical compounds to establish whether they possessed the capability according to the claim; this represented for the skilled person an invitation to perform a research programme and thus an undue effort (following T 435/91). See also T 1140/06.
If the patent claims require that a specific aim should be achieved (in this case, that a specific value of a parameter should not be reached), then there is no guarantee that the invention can be carried out in accordance with Art. 100(b) EPC if the patent affords the skilled person no clue as to how he can achieve this aim outside the scope of the embodiments without an undue burden of research (T 809/07).