## 3 Method for Proactive Management of Requirement Changes
### Change Risk Assessment
#### 3.2.2 Requirement Change Impact Assessment  
For requirement change impact assessment, the influencing factors need to be differentiated (cf. Figure 4). The starting point is an initial change request from an exogenous change initiator (for instance, a customer). Implementing this change leads to an impact directly associated with changing the affected requirement (local change impact), but also sends out subsequent change impulses to other requirements (consecutive change effects). Those exist due to requirement dependencies and need to be implemented to ensure consistency and validity of the requirement set. Usually, changes caused by consecutive effects are referred to as change propagation [5,7,9]. By adding up the local change impact of all requirements on a propagation path, the collective change impact of the initial change request can be assessed [55]. To assess the collective change impact, the consecutive effects are determined first (cf. Figure 5). This is implemented automatically based on the results from the dependency analysis. Dependencies are weighted by their type to indicate propagation behavior and requirements are weighted by priority to indicate local change impact. As a second step, the weighted requirement network is analyzed by means of a modified PageRank algorithm to identify critical requirements. Those are selected and evaluated regarding collective change impact manually by experts as a last step. In the following, the three steps to assess the change impact are explained in detail. The weighting of dependencies and requirements is carried out in two steps. First, the dependencies are weighted to estimate the consecutive effects of a change in requirements. Then, the requirements are weighted based on the expected local effects. The evaluation of the consecutive effects of a requirement change is based on the propagation behavior of a certain requirement dependency. The existence of a dependency is a prerequisite and determines whether a requirement can be affected by the original change. The type of dependency indicates the propagation behavior between two requirements. In order to identify dependencies which can lead to a change propagation, dependency types from existing reference models [56; 57; 58] are considered. Three categories are derived: mandatory propagation, possible propagation and no propagation are classified. The differentiation is based on previous studies on propagation behavior by dependency type [59; 60]. Since those were made in the context of software development, the validity for interdisciplinary systems was checked based on a requirement set of an example system (drone). Starting with the assumption that all dependencies have mandatory propagation behavior, this assumption tried to be falsified for each dependency type by finding exemplary constellations without mandatory propagation. This was repeated for not mandatory propagation. The results were consistent with the findings from software development, so that the following categorization of dependency types was used to evaluate the consecutive effects of a requirement change:  
* Mandatory propagation behavior: requires, is required by and is refined by
* Not mandatory propagation behavior: positive/negative correlation
* No propagation behavior: refines, cost and value  
The weighting of the dependency types of a category is based on the propagation likelihood. For the category mandatory propagation this is 100% (weighting = 1) and for the category no propagation this is 0% (weighting = 0). The weighting of the category not  
Figure 4: Basic elements of change impact analysis [55]. Figure 5: Overview of the steps for assessment of requirement change impact. mandatory propagation can be determined context specifically depending on risk affinity or static data on the propagation likelihood. As an example, a weighting of 0.5 is used.