This approach is illustrated using a requirement drawn from the air traffic domain (Figure 8.7). The abstract goal that the system should visualise air traffic scenarios in real time. This motivates a system requirement that it should perform in real time. This motivates to non-functional requirements that are mainly concerned with performance and capacity, which in turn motivates quantitative non-functional requirements. For example, the non-functional requirements that’ radar data should be displayed in real time’ motivates the more detailed non-functional requirements’ aircraft position should be displayed in less than 0.165 of radar sweep period’. Further analysis of the non-functional requirements results in successively more specific requirement statements represented as the leaves of the diagram shown in figure 8.7.