## Materials and Methods
### Conceptual Model of Requirement Change Risk Factors  
So far, no holistic model of risk factors exists due to the lack of attention for risk management of requirement changes. The existing approaches focus on either one or both of the two basic dimensions of risk: Likelihood of change and impact of change. This is adequate for rough expert estimations but needs to be detailed for a decisive risk assessment as a basis of risk management. Risk factors need to be unambiguous and independent to be analyzed and assessed systematically. Therefore, the risk dimensions likelihood and impact are broken down into its underlying risk factors. To identify risk factors that influence the likelihood of change, the root causes of a change need to be analyzed. Root causes trigger changes and therefore constitute the change likelihood. Studies show that many requirement changes are triggered by the solution space [40; 41; 42]. Although EC and requirement change are differentiated, there still is a close dependency between them. On the one hand, requirement changes can affect the boundaries of solution space and thereby lead to EC. On the other hand, requirement changes can be triggered by the solution space, for instance, design improvements or increased understanding of the technical solution [20]. Requirement changes can also come from the evolution or change of stakeholder needs (for instance, customer preferences and priorities), which directly affect the problem space [43; 44; 45]. A systematic review on causes of requirement changes from [PERSON] and [PERSON] revealed that besides the interconnection of problem and solution space there are four additional categories of change causes [20]:  
1. External market (including stakeholders like customers/users, government bodies and competitors). 2. Customer organization (changes impact the needs of the customer and as a result, impact the design and requirements). 3. Project vision (better understanding of the problem space from a customer point-of-view and the emergence of new opportunities and challenges). 4. Requirement specification (developer's point-of-view and their improved understanding of the problem space and resolution of ambiguities related to requirements). 5. Solution (related to the solution of the customer's requirements and the techniques used to resolve this). In addition to causes of an initial requirement change, change propagation can also lead to requirement changes. The importance of propagation effects as a subsequent cause for requirement changes is stressed, for instance, by a long lasting but clearly documented case study on the development of a complex sensor system in aerospace industry with 41,500 change requests over a period of eight years [8]. The study shows that in parts, up to 50% of requirement changes were triggered by other requirement changes. To capture those, the current understanding of requirement change causes is enriched by the two dimensions exogenous and endogenous. Exogenous change causes trigger an initial requirement change. Those are not related to requirement interplay and come from outside the requirement set. Within the exogenous change causes, the more common differentiation of external and internal causes is used to categorize causes from inside the organization (internal; for instance, incomplete system understanding) or outside the organization (external; for instance, volatile customer needs). Endogenous changes can only occur after an initial change was triggered and come from the propagation effects between requirements. The overall concept of requirement change causes is illustrated in Figure 2.  
Looking at the impact of change, there is no in-depth analysis of risk factors published yet. Still, the two basic risk factors of change impact can be derived from approaches for change impact analysis (for instance [9; 19; 46]). On the one hand the incoming change request and on the other hand the change behavior of the affected requirement influences the change impact.