Characterizing the impact of requirements volatility on systems engineering effort

This paper describes the results of a study aimed at increasing the understanding of the causes of requirements volatility, its impact on systems engineering effort, and its changing dynamics over the system life cycle. The objective of the research is to improve the ability of systems engineers to anticipate and manage the effects of changing requirements. The ultimate goal of this study is to develop a model for quantifying the impact of requirements volatility on systems engineering effort that can be incorporated into the Constructive Systems Engineering Cost Model. Based on a review of the literature and expert judgment collected through surveys in five workshops, we identify five observations that summarize the key considerations of requirements volatility. First, a set of project organizational, technical, and contextual factors were ranked by subject matter experts in terms of their influence on requirements volatility. Their responses point to poor initial understanding of the system and customer needs as the leading cause of requirements volatility. Second, our results suggest that, while volatility tends to decrease over time, the number of requirements changes may increase during transitions between life cycle phases. Third, requirements volatility increases the functional size of the project and causes rework of engineering products, driving an increase in systems engineering effort. Fourth, the effect of requirements volatility on systems engineering effort increases the later the change occurs in the system life cycle. Fifth, the effort impact of a requirements change varies depending on the type of change (added, deleted, and modified).