The assessment of software systems often requires to consider together qualitative and quantitative aspects. Because of the different nature, measures belong to different domains. The main problem is to aggregate such information into a derived measure able to provide an overall estimation. This problem has been traditionally solved trough the transformation of qualitative assessments into quantitative measures. Indeed, such a transformation implicitly assumes a conceptual equivalence between the terms quantitative and objective on one side, and qualitative and subjective on the other side. An alternative approach is to consider logical aggregation models, able to infer the overall evaluation based on the assessment of individual attributes. This approach requires an early transformation of quantitative measures in qualitative assessments. Such a transformation is possible trough the use of judgment functions. The aim of this paper is to introduce the judgment functions and to study their properties.