Which qualitative research method involves detailed interviews to gather deep insights?

The method that involves detailed interviews to gather deep insights is in-depth interviews. This qualitative research approach is designed to explore a participant's thoughts, feelings, and experiences in a comprehensive manner. By conducting one-on-one interviews, researchers can engage with participants on a personal level, allowing for a deeper understanding of their perspectives and motivations.

In-depth interviews are characterized by open-ended questions that encourage participants to express themselves freely, often leading to rich, nuanced data. This method tends to produce detailed qualitative data that can uncover themes, patterns, and unique insights that may not emerge from less personal or structured data collection techniques.

The other options, while relevant in qualitative research, do not focus primarily on the method of in-depth personal interviews. Focus groups involve a group discussion format, case studies examine specific instances or phenomena typically across multiple data sources, and surveys primarily utilize structured questionnaires to gather data from a larger number of respondents, which limits the depth of individual insights.