The quantitative approach to morphological productivity developed by Baayen and collaborators is crucially based on the count of hapax legomena in a given, very large textual corpus. In this paper, Baayen's main idea is applied to the little explored domain of Italian prefixation, on the basis of a 75,000,000-token newspaper corpus, and a significant improvement of his procedure is proposed by calculating productivity values at equal token numbers for different affixes. Consequently, variably-sized subcorpora must be sampled to compare affixes displaying different token frequencies. Following this approach, the Italian productive prefixes ri- and in- can be ranked by productivity within their respective derivational domains, and the impact of different derivational cycles on the measure of productivity can be dealt with satisfactorily.