On the Distinction of Functional and Quality Requirements in Practice

TL;DR

This study investigates how practitioners differentiate between functional and quality requirements in practice, revealing significant process differences and highlighting the importance of conscious decision-making regarding this distinction.

Contribution

It provides empirical insights into the practical handling of FRs and QRs, emphasizing the need for conscious decisions and awareness of associated risks.

Findings

Development processes differ significantly for FRs and QRs

Practitioners' reasons for distinction are often expectation-based

Making the distinction consciously can mitigate risks

Abstract

Requirements are often divided into functional requirements (FRs) and quality requirements (QRs). However, we still have little knowledge about to which extent this distinction makes sense from a practical perspective. In this paper, we report on a survey we conducted with 103 practitioners to explore whether and, if so, why they handle requirements labeled as FRs differently from those labeled as QRs. We additionally asked for consequences of this distinction w.r.t. the development process. Our results indicate that the development process for requirements of the two classes strongly differs (e.g., in testing). We identified a number of reasons why practitioners do (or do not) distinguish between QRs and FRs in their documentation and we analyzed both problems and benefits that arise from that. We found, for instance, that many reasons are based on expectations rather than on evidence. Those expectations are, in fact, not reflected in specific negative or positive consequences per se. It therefore seems more important that the decision whether to make an explicit distinction or not should be made consciously such that people are also aware of the risks that this distinction bears so that they may take appropriate countermeasures.