## 4 Results
### Method for Systematic Assessment of Requirement Change Risk
#### 4.1.2. Consideration of Change Propagation Impact and Endogenous Change Likelihood
By considering generic main features rather than each specific requirement, the application effort was reduced from an exponential correlation of the number of requirements and assessment effort (pairwise assessment) towards a one-time effort with disproportionately low assessments to be made. Similar to the categorization by influence factors, the degree of generalization within the rule set can be determined individually to suit the project characteristics. Additionally, by generalization the dataset on interrelations can be transferred to similar development projects, further reducing the application effort beyond the initial use. The generalization--though potentially reducing accuracy--was judged positively by the case study partners as the potential benefit of reduced effort succeeded the potential loss of accuracy. This assessment was made with participants from all industrial partners. Practitioners agreed that application effort of individual assessment would be too high for industrial application, if the requirement set exceeds about 10 requirements (exponential growth). Therefore, generalization was considered as useful to reduce application effort and maintain sufficient accuracy of results. This estimation was later confirmed as part of the evaluation (Section 5). The rule set is shown in Figure 6 below. Figure 6: Rule-set for semi-automatized assessment of requirement interrelations [51]. Given the semi-automatized assessment of requirement interrelations, unconnected elements in the requirement list were transferred into a RSM [51]. The transfer process is shown in Figure 7 below. Requirements were structured according to correlated main features (structured requirements list). This list was used as an input for semi-automatized assessment of requirement interrelations. Results are presented to the user for check-up and adaptation if required. Based on the RSM, interrelations between requirements can be analyzed. Graph-theoretical approaches such as active and passive sum are capable for the characterization of requirements [14; 24; 48; 62]:  
* By the Active Sum (AS), the impact of a single requirements on other requirements is described. * By the Passive Sum (PS), the potential impact from other requirements on a single requirement is described. In order to classify the change propagation impact, both AS\({}_{\text{i}}\) and PS\({}_{\text{i}}\) of a requirement \(\text{r}_{\text{i}}\) are considered for classification based on the average AS (\(\OAS\)) and PS (\(\OPS\)), see Table 4 below. Three classes among requirements were differentiated: Within Class I, all requirements with an above average AS were clustered. These were considered as most important due to their driving nature for change propagation. Within the second class, Class II; all requirements with an above average PS, but below average AS were clustered. These requirements were likely to be influenced by other requirements (mostly from Class I) and therefore, potentially, have a high endogenous change likelihood. Class III contained requirements with both AS and PS below average. This means minor influence on other requirements. For increased precision in classification, a dependency score is calculated based on the classification described. Therefore, the following assignment of numerical values on the interval [0, 3] shown in Table 5 is defined:  
For a more detailed differentiation within the numerical boundaries, the assignment of rational numbers was done using the Euclidean distance of a requirement \(\text{r}_{\text{i}}\) specified by the coordinate values \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**AS**} \\ \hline
**PS** & **AS\({}_{\text{i}}\geq\OAS\)** & **AS\({}_{\text{i}}\) \(<\OAS\)** \\ \hline PS\({}_{\text{i}}\geq\OPS\) & Class I & Class II \\ PS\({}_{\text{i}}<\OPS\) & Class I & Class III \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Classification of requirements according to Active Sum (AS) and Passive Sum (PS).