Requirements models also have the advantage that the

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Requirements models also have the advantage that the different types of modeling elements within the same modeling language dictate the method of abstraction as well as what is being abstracted and what is not. 6.1.5 Combined Use of Models and Natural Language Using both natural language and requirements models in combination allows the advantages of both documentation techniques to be exploited while minimizing their disadvantages. For example, natural language requirements can be summarized and their interrelations depicted using models. On the other hand, natural language can help enrich requirements models and modeling elements with additional information. 6.2 Goal Models Many methods in requirements engineering are based on the explicit con- sideration of stakeholders’ intentions by means of goals (e.g., [van Lam- sweerde et al. 1991] and [Yu 1997] ). Ordinarily, the effort required to explicitly consider goals during requirements engineering is minimal. However, the positive impact on requirements engineering—if goals are modeled—and on the quality and comprehensiveness of the requirements is very high. Goals are a stakeholder’s (e.g., a person’s or an organization’s) description of a characteristic property of the system to be developed or the development project. Natural-language-based and model-based documentation Goals are very well suited to refine the vision of the system. Refining a goal is known as goal decomposition. Goals can be documented using natural language (e.g., by means of predesigned templates) or using goal models. A widely known and very common goal modeling technique is the use of AND/OR trees. By means of AND/OR trees, hierarchical decompositions can be documented. The type of refinement relation is depicted by graphic representations of the branches. The direction of the goal decomposition is not represented through branches but through the top-down structure of the tree.
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6.2 Goal Models 63 6.2.1 Goal Documentation Using AND/OR Trees Using AND/OR trees, two types of decomposition relationships can be dis- tinguished. Figure 6-1 schematically shows these types of decomposition as well as their modeling elements. Figure 6-1 Modeling of goal decomposition using AND/OR trees AND-decomposition vs. OR-decomposition With regard to decomposition relations, one can differentiate between AND-decomposition and OR-decomposition. In case of AND-decompo- sition, every sub-goal must be fulfilled so that the super-goal (the root) is fulfilled. In contrast, in OR-decomposition, it suffices if at least one sub- goal is fulfilled so that the super-goal is met. 6.2.2 Example of AND/OR Trees Figure 6-2 shows an AND/OR tree that documents the hierarchical decomposition of the goal “Comfortable navigation to destination”.
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