## 3 State of the Art
### Requirements Management
#### 3.1.2 Stages in Requirements Management  
In order to structure the later discussion in this paper, a model was developed on the basis of requirements engineering process models (compare e.g., [23; 24]), which allows for distinguishing different stages in requirements management (Figure 5).  
The main stages are briefly explained in remaining part of this sub-section.  
A starting point of the requirements management process might be the collection of requirements, though this step may be repeated again and again during the process of product development. The collection of requirements may include an application of methods such as user surveys, benchmarking, or quality function deployment (QFD). In this stage, the voice of the customer plays an extremely important role (compare e.g., [19]). It is important to note that requirements specifications can never fully cover all aspects of an envisaged product [25].  
The notion \"structuring of requirements\" may include activities which are intended to arrange larger lists of requirements into some kind of sensible structure. The structuring of requirements is important, because the collection of requirements for complex systems may be vast, and an overview and handling of the large set of requirements usually requires some structure, e.g., the product structure.  
Several literature sources recommend a classification of requirements; important aspects may be the general type (functional requirement, quality requirement, or structural requirement), the solution dependence (goal oriented requirements, scenario-oriented requirements or solution oriented requirements), or the abstraction level (compare [PERSON] and [PERSON] [16]). Furthermore, [PERSON] et al. [14] and [PERSON] and [PERSON] [22] also present structures for classification.  
Assuring the consistency of requirements includes activities which are intended to find contradictions between requirements (these contradictions need to be resolved) and to identify competitive requirements. In industrial practice, these activities are extremely important [1] and should be started early.  
Prioritizing Requirements  
In the case of competitive requirements, a prioritization of requirements is necessary. Here, it is crucially important to bring up conflicts and to find and document compromises, especially with respect to industrial practice [1].  
Figure 5: Stages in the requirements management process cycle.  
The tracking of requirements (compare requirements traceability [26]) is a central element of requirements management. It is immensely important that the requirements can be clearly identified during the whole product development process, and that a version and change management process for requirements is in place [16].  
One might argue that the testing of requirements is a major part of a product development project and cannot be seen as part of the requirements management. It is, however, a very important aspect of requirements management that these crucial activities are supported in an optimum manner, and that the requirements are readily available in a consistent, plausible form for the analysis activities.  
The structured documentation of requirements is a main challenge because usual requirements management systems lack an updated representation of the product structure.