## 3 Method for Proactive Management of Requirement Changes
### Change Risk Assessment
#### 3.2.3 Requirement Change Likelihood  
Requirement change likelihood is composed of the exogenous and endogenous change likelihood (cf. Figure 9). Exogenous change initiators trigger an initial requirement change. Those are not related to requirement interplay and come from outside the requirement set. Endogenous changes can only occur after an initial change was triggered and originate from the propagation effects between requirements [14]. To determine the requirement change likelihood, exogenous and endogenous change likelihood are calculated and merged towards the overall change likelihood (cf. Figure 10). To calculate the exogenous change likelihood, historical change data are analyzed. To ensure the validity of historical change data for the requirements set at hand, they are narrowed down to equivalent circumstances. This is implemented by the manual assignment of relevant change initiators as well as reference projects with similar characteristics. The endogenous change likelihood can be calculated based on the requirement dependencies and their propagation behavior. This step is fully automated and based on a modified PageRank algorithm. In the following, the three steps to calculate exogenous, endogenous and overall change likelihood are explained in detail. Figure 10: Overview of steps for the assessment of requirement change likelihood. Figure 9: Overview of exogenous and endogenous change causes [14]. Exogenous changes are triggered from outside the requirement set. Therefore, it is necessary to assess how likely a change of a certain requirement will receive a change impulse by an environmental factor. At first, environmental factors need to be identified that can potentially send out a change impulse. In the following, those environmental factors are called change initiators. Second, it needs to be assessed how likely a change impulse will be sent out by a change initiator. Historical data on the number of requirement changes triggered by an initiator are seen as the best indicator for this. Still, this cannot be used to determine an absolute change likelihood, since this is influenced by many context specific factors (for instance, type of change impulse or available resolution alternatives for a change impulse). It is also not valid to forward the change likelihood from an initiator to all assigned requirements due to conditional probability. In consequence, the aim is to calculate a characteristic value for the exogenous change likelihood, which indicates, how strong a requirement is affected by exogenous change impulses. 1. Assigning requirements to change initiators  
The first step is to identify change initiators, where an initial change impulse addressed to a certain requirement can come from. Studies differentiate five areas of exogenous change initiators: external market, customer organization, project vision, requirement specification and solution [13; 66]. In discussion with practitioners those are judged as too generic and therefore are further detailed into 13 initiators (based on [67]):  
* external market: politics (such as regulations), suppliers (such as market stability) and market including further stakeholders (such as response to competitors),
* customer organization(s): customer (such as strategy) and customer organizational structure (such as reorganization or hardware/software changes),
* project vision: business case (such as cost overrun), technology (such as new opportunity) and knowledge (such as a new stakeholder),
* requirement specification (such as increased understanding): requirements engineer and project leader,
* solution (such as design improvement): software development, hardware development and production. Based on the requirement's source and content, relevant initiators from which a change impulse can be sent out are manually assigned to requirements by experts.