We investigate how probabilities can be assigned to dispositions in ontologies, building on Popper's propensity approach. We show that if D is a disposition universal associated with a trigger T and a realization R, and d is an instance of D, then one can assign a probability to the triplets (d,T,R) and (D,T,R). These probabilities measure the causal power of dispositions, which can be defined as limits of relative frequencies of possible instances of T triggering an instance of R over a hypothetical infinite random sequence of possible instances of T satisfying certain conditions. Adopting a fallibilist methodology, these probability values can be estimated by relative frequencies in actual finite sequences.