Non-functional requirements as qualities, with a spice of ontology

We propose a modeling language for non-functional requirements (NFRs) that views NFRs as requirements over qualities, mapping a software-related domain to a quality space. The language is compositional in that it allows (recursively) complex NFRs to be constructed in several ways. Importantly, the language allows the definition of requirements about the quality of fulfillment of other requirements, thus capturing, among others, the essence of probabilistic and fuzzy goals as proposed in the literature. We also offer a methodology for systematically refining informal NFRs elicited from stakeholders, resulting in unambiguous, de-idealized, and measurable requirements. The proposal is evaluated with a requirements dataset that includes 370 NFRs crossing 15 projects. The results suggest that our framework can adequately handle and clarify NFRs generated in practice.