Are "Non-functional" Requirements really Non-functional?

TL;DR

This paper empirically investigates whether non-functional requirements are truly non-functional, finding that most describe system behavior and could be treated similarly to functional requirements.

Contribution

It provides an empirical analysis of 530 requirements, challenging the traditional distinction between functional and non-functional requirements.

Findings

Most NFRs describe system behavior

Many NFRs can be handled like functional requirements

NFRs are often not as non-functional as traditionally thought

Abstract

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.