The quantification of so-called external software attributes, which are the product qualities with real relevance for developers and users, has often been problematic. This paper introduces a proposal for quantifying external software attributes in a unified way. The basic idea is that external software attributes can be quantified by means of probabilities. As a consequence, external software attributes can be estimated via probabilistic models, and not directly measured via software measures. This paper discusses the reasons underlying the proposals and shows the pitfalls related to using measures for external software attributes. We also show that the theoretical bases for our approach can be found in so-called ldquoprobability representations,rdquo a part of Measurement Theory that has not yet been used in Software Engineering Measurement. By taking the definition and estimation of reliability as reference, we show that other external software attributes can be defined and modeled by a probability-based approach.