De Biagi et al. (2017) have proposed a procedure for building a fallen block volume-frequency law for rockfall phenomenon. The input data are from both the recording of rockfall events and from the survey of fallen block volumes. The epistemic and aleatoric uncertainties present in the approach affect the value of the parameters of the law. It is shown how to quantify the errors due to missed events, to an observation period of finite duration and to a limited set of measured blocks. At the end, the procedure outputs corrective parameters to compute a design volume for rockfall analysis and engineering calculations.