In addition to the classification into functional

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limit the solution space available during the development process. In addition to the classification into functional requirements, quality requirements, and constraints, a number of different classifications of requirements are used in practice. For example, there are a number of clas- sifications suggested by several standards, e.g., CMMI [SEI 2006] or SPICE [ISO/IEC 15504-5]. Other classification schemes describe requirement attributes, such as the level of detail of a requirement, the priority, or the degree of legal obligation of requirements (see chapters 4 and 8 ). Definition 1-4: Functional Requirement A functional requirement is a requirement concerning a result of behavior that shall be provided by a function of the system. Definition 1-5: Quality Requirement A quality requirement is a requirement that pertains to a quality concern that is not covered by functional requirements. Definition 1-6: Constraint A constraint is a requirement that limits the solution space beyond what is necessary for meeting the given functional requirements and quality requirements.
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1.5 Importance and Categorization of Quality Requirements 9 1.5 Importance and Categorization of Quality Requirements In daily practice, quality requirements of a system are often not docu- mented, inadequately documented, or improperly negotiated. Such cir- cumstances can threaten the project’s success or the subsequent acceptance of the system under development. Therefore, the requirements engineer should place special emphasis on the elicitation, documentation, and negotiation of quality requirements during the development process. Typically, many different kinds of desired qualities of the system are assigned to the requirement type quality requirement . In order to be able to deal with quality requirements in a structured manner, many different classification schemes for quality requirements have been proposed. The ISO Standard 9126 ( [ISO/IEC 9126] ), for example, suggests a classification scheme for quality requirements that can also be used as a standard struc- ture for requirements documentation and as a checklist for requirements elicitation and validation. The following list includes some typical catego- ries: Requirements that define the quality of system functions, in particular with regard to appropriateness, security and safety, accurateness of cal- culations, interoperability, and respective conformity to standards Requirements that define the dependability of functionalities, in par- ticular with regard to robustness, fault tolerance, and recoverability Requirements that define the usability of a system, in particular with regard to understandability, learnability, and ease of use Requirements that define the system efficiency, in particular with regard to time behavior (e.g., computation time) or consumption behavior (e.g., resource utilization) Requirements that define the changeability of a system, in particular with regard to analyzability, changeability, stability, and testability Requirements that define the portability of a system, in particular with regard to adaptability, installability, and respective conformity to
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