Control systems deployed in commercial aircrafts, emergency systems installed in nu- clear power plants, remote-controlled surgery robots in hospitals, and automated brak- ing assistants in automobiles are just a few examples of modern safety-critical systems. During the last decades, humans are increasingly poised to hand the responsibility for their lives over to electronic systems.
The growing complexity of these systems, and the fact that a failure of a single sub- system may have fatal consequences for the users, forces the industry to reconsider the underlying development process to obtain products of the required quality.
The large scale of the systems inevitably requires a great number of stakehold- ers. Communication of needs and constraints is thus fundamentally complex, both be- cause of the sheer amount, and even more because of different—domain specific— vocabularies. Ambiguities in early specifications (Figure 1) then yield costly changes at later project phases.
To overcome such issues, requirements based approaches (cf. ) are used in sys- tems development. Requirements describe properties of a system. They give specifica- tions of varying precision of the system to be developed, and later provide means to judge whether the product meets the goals.
The IEEE Standard 830  lists desirable characteristics of requirements: (a) cor- rect, (b) unambiguous, (c) complete, (d) consistent, (e) ranked in importance and/or stability, (f) verifiable, (g) modifiable, and (h) traceable. Further, each requirement shall be uniquely identified.
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