## 4 Experimental Results
### Addressing RQ2: Impact of PR Behaviors on Contributor Engagement in GitHub Projects
#### 4.3.1 Merge Experience and the Influence of p1 on Merges  
In this section, we analyze the impact of merge experiences on participant engagement and the specific role of the first project participant (p1) in managing merges. Pull request (PR) merges are a critical step in open-source collaboration, and a participant's experience of having their PR merged can significantly influence their continued participation. Figure 8 shows the SHAP value results of the independent variables in this regard.  
Figure 8a shows the SHAP dependence plot for the merge variable, representing the participant's experience with PR merges. The graph clearly indicates that when participants have no merge experience (values close to 0), the impact on their participation duration is negative. This suggests that participants who never get their PRs merged are likely to reduce or stop their contributions over time. However, the moment participants gain merge experience, there is a sharp increase in the SHAP value, indicating a significant positive influence on their engagement. participants with merged PRs tend to stay active for longer periods, which underscores the importance of successfully merging their contributions. Moreover, participants with higher k values--those more central to the project's  
Figure 8: SHAP Analysis of merge and p1_merge.  
network--are often among those who have had their PRs merged. This highlights the correlation between merge experiences and a participant's increasing role in the project's network structure, reinforcing their engagement and participation.  
Figure 8b presents the PDP for the variable p1_merge, which represents the likelihood that the first participant (p1) merges PRs submitted by others. The analysis indicates that when p1 does not manage PRs (p1_merge values close to 0), SHAP values are slightly negative, implying a negative impact on the participation duration of participants whose PRs are not reviewed or merged by p1. Conversely, when p1_merge values approach 1, SHAP values become positive, indicating that PR management by p1 positively influences the participation duration of contributors. This effect is particularly noticeable for participants on the periphery of the network (represented by yellow dots), as their engagement appears more dependent on p1's actions. In contrast, participants with higher k-values (located near the network core) show a more diverse distribution of SHAP values, suggesting that their participation is less influenced by p1's PR management. This finding reflects the varying levels of dependence participants have on project leaders based on their network roles and centrality.  
These two variables collectively demonstrate how collaboration and merge activities contribute to the sustainability of project participation. Specifically, participants who actively manage PRs or are supported by active PR management--whether by core participants or p1--are more likely to stay engaged over time. This analysis also underscores the significance of establishing effective merge strategies and fostering distributed collaboration to ensure the long-term retention of participants. By optimizing PR review and merge processes, project maintainers can create an environment that promotes sustained engagement across all levels of participants.