Are “Non-functional” Requirements really Non-functional?

Abstract

Non-functional requirements (NFRs) are commonly distinguished from functional requirements (FRs) 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 FRs, without quantitative mea- sures, 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 FRs. In this paper, we empirically investigate this point of view and aim to increase our understanding on the nature of NFRs addressing system properties. Our re- sults 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 FRs.