Context: Requirements Engineering (RE) research has established a common agreement on the impact that the quality of requirements has on subsequent software development activities and artifacts. Furthermore, empirical investigations suppose that RE quality defects tend to scale in cost for remediation when left unattended. This motivates the need for requirements quality assurance.

Problem: This need has been met with requirements quality research, which abounds with publications proposing writing rules and guidelines that are meant to ensure requirements of high quality. However, recent studies have questioned the rigor and relevance of these publications, which would undermine the practical applicability of requirements quality research: requirements quality is a means to an end and serves a specific purpose (i.e., minimizing the emitted risk on downstream activities), but when this purpose is not met due to lack of a rigor and practical relevance, the approach to researching requirements quality needs to be rethought.

Aim: The notion of good-enough requirements engineering constitutes a context-sensitive, activity-based perspective on requirements quality. In this thesis, we aim at both (1) understanding and (2) exploring possibilities of operationalizing this notion.

Methods: We employ a mixed-methods approach to achieve our aim. We use theory adoption in order to provide a theoretical foundation for requirements quality research, conduct a survey to understand the level of theory adherence in the requirements quality literature, and perform subject-based classification to generate an overview of theory-related elements proposed in literature. 

Results: Through theory adoption we derive a harmonized, activity-based requirements quality theory that frames requirements quality according to its impact on subsequent activities and hence ensures its relevance. The subsequent survey confirms that there is a lack of rigor and relevance in previous requirements quality publications, which likely explains the lack of adoption of the research in practice. The overview of quality factors in a subject-based classification is a first step to centralize requirements quality research for visibility and effective reuse.

Conclusion: The notion of good-enough requirements engineering has the potential to re-focus requirements quality research on a more profound notion of rigor and relevance. In this thesis, we report on a first requirements quality theory. Through adherence to this requirements quality theory and contribution to the central repository of subject-based classification, the operationalization of the concept of good-enough requirements engineering can effectively support predicting the impact that requirements quality has on subsequent software development activities in the future.

Background: High-quality requirements are considered crucial for successful software development endeavors as the requirements' purpose is to inform subsequent activities like implementation or testing. Requirements quality defects have been shown to incur significant costs for remediation, scaling up even to project failure. At the same time, the effort to improve the quality of requirements must be justified. Organizations developing software, therefore, need to understand when their requirements artifacts are of "good enough'' quality, i.e., they need to be able to identify the optimum between over- and under-engineering.

Problem: The body of knowledge in requirements quality does not yet offer solutions that would allow organizations to identify that optimum due to three shortcomings: (1) there is no generally accepted, theoretical foundation to describe requirements quality that can serve as a basis to coordinate distributed research efforts and the synthesis of evidence in the field, (2) the scientific practice currently applied in the field is of limited rigor to draw reliable conclusions from existing empirical contributions, and (3) the field lacks empirical evidence that can be aggregated to form a holistic view of requirements quality. These are potential causes for the lack of adoption of requirements quality research in practice.

Goal: In this cumulative, publication-based thesis, we address these three shortcomings and aim to contribute to a more evidence-based approach to requirements quality research grounded in scientific theory.

Method: First, we develop a theoretical foundation by adopting and integrating existing software engineering theories. Second, we evaluate the state of the art of data analysis and open science in the field and provide guidelines to improve these practices. Third, we demonstrate the application of these guidelines and conduct a controlled experiment to contribute additional empirical evidence to the field.

Results: The resulting set of analytical theories specifies requirements quality and provides a structure for future empirical contributions. Our evaluation of the state of the art shows both the need for a common theoretical foundation as well as support for applying rigorous research practices. Our empirical studies contribute to these needs and illustrate the complexity of the impact that requirements quality defects have on subsequent activities. Finally, we develop a method for the effective aggregation of empirical results.

Conclusion: Our theoretical, methodological, and empirical contributions help to coordinate a productive and constructive research agenda on requirements quality that is based on evidence and grounded in theory. This allows for rigorous and practically relevant research that ultimately informs organizations on how to engineer good-enough requirements.