*Quality Criteria of a Case Study*

To ensure the high quality of our case study research, we considered several quality criteria for conducting qualitative case studies according to Yin [40], e.g., objectivity, reliability, internal, external, and construct validity.

First of all, the objectivity and reliability of a case study should be guaranteed to enable the reproducibility of the case study. Transparency and controllability of the scientific procedure are essential characteristics of reliable research. For this purpose, a research protocol was prepared, the case study data (e.g., transcripts) were archived, and the implementation of the case study was described in detail [40].

The case study validity is divided into construct validity, as well as internal and external validity. Construct validity ensures the choice of the correct measures for answering the research questions [40]. The interviewees received a summary of the interview guideline to prepare for the interview. Additionally, a non-disclosure agreement was signed to ensure that interviewees could freely disclose information without fearing social or professional consequences. During the development of the interview guide, a wide variety of already

existing scientific knowledge was used as a basis for the research to enhance construct validity. Further, the interviews were conducted with nine organizations in the same supply chain, allowing mutual data triangulation. Possible biases in the subjective expert perceptions could be identified, and any contradictions between the interviewees' answers could be clarified, e.g., through inquiries after the interviews.

Internal validity is characterized by the logical presentation of the cause–effect relationships in the research data [40,42]. Patterns in the data should be compared systematically and transparently, which was granted in the study by structuring the data analysis using a coding system. Furthermore, contradictions in the research data were searched for to ensure internal validity. External validity refers to the generalizability and transferability of the findings. Even though case studies are not intended to provide empirical evidence in research fields but are exploratory, we emphasized case selection to enhance external validity [40]. We chose a supply chain with nine actors extended over several countries and continents to best represent the challenges of governance across multiple supply chain stages. Additionally, the selected supply chain already provides sustainable end products (confectionaries), which allows the analysis of a consistently sustainable supply chain.
