Case study requires multiple sources of evidence. However, interviews was the only source indicated by the description. I am curious as to what other data will be collected.
We collected data from documents provided by the U.S. Army Chaplain Center and School and participants, as well as Army regulations. The documents described pastoral care principles and guidelines.
Triangulation of sources of qualitative data is very traditional approach. You can use mini-survey, focus group discussion, direct observation, desk review and content analysis. If you have enough resources you may use survey.