text: Non-functional requirements (NFRs) are commonly distinguished from functional requirements by differentiating how the system shall do something in contrast to what the system shall do. This distinction is not only prevalent in research, but also influences how requirements are handled in practice. NFRs are usually documented separately from functional requirements, without quantitative measures, and with relatively vague descriptions. As a result, they remain difficult to analyze and test. Several authors argue, however, that many so-called NFRs actually describe behavioral properties and may be treated the same way as functional requirements. In this paper, we empirically investigate this point of view and aim to increase our understanding on the nature of NFRs addressing system properties. We report on the classification of 530 NFRs extracted from 11 industrial requirements specifications and analyze to which extent these NFRs describe system behavior. Our results suggest that most "non-functional" requirements are not non-functional as they describe behavior of a system. Consequently, we argue that many so-called NFRs can be handled similarly to functional requirements.
  @cite_19: Although Non-Functional Requirements (NFRs) are recognized as very important contributors to the success of software projects, studies to date indicate that there is still no general consensus in the software engineering community regarding the notion of NFRs. This paper presents the result of an extensive and systematic analysis of the extant literature over three NFRs dimensions: (1) definition and terminology; (2) types; and (3) relevant NFRs in various types of systems and application domains. Two different perspectives to consider NFRs are described. A comprehensive catalogue of NFRs types as well as the top five NFRs that are frequently considered are presented. This paper also offers a novel classification of NFRs based on types of systems and application domains. This classification could assist software developers in identifying which NFRs are important in a particular application domain and for specific systems.
summary: @cite_19 perform a literature review on NFRs, investigating the notion of NFRs in the software engineering literature to increase the understanding of this complex and multifaceted phenomenon. Amongst others, they found about 114 different NFR classes. As a result of a frequency analysis, they found that the five most frequently mentioned NFR classes in literature are performance , reliability , usability , security , and maintainability (in that order). In our study, we got similar results: we found that the five most frequently used NFR classes in our industrial specifications are security , reliability , usability , efficiency , and portability (in that order). We excluded functionality from this list, as it is not a classical NFR class . While performed their analysis on available literature, our study analyzes NFRs documented in industrial projects.