In-Depth Interviews (IDIs), which are mainly qualitative, may also be quantitative. This method allows them to ask for quantitative information while also conducting interviews that are individualized to each respondent using a structured questionnaire.

This format can be helpful for sensitive topics, multifactorial or non-obvious decision makers, or populations that are more difficult to reach with traditional surveys. The structure of the interview allows the interviewer to remain consistent, as all questions are predetermined and largely consist of closed questions or the use of rating scales making the data quantifiable and thus comparable across interviews.

These key informant interviews make sense in the context of healthcare research, executive interviews, or niche B2B industries where the insights of a few selected individuals can provide quality information. For example, in an IDI, a doctor may be asked to provide a Likert scale response to the question of how successful treatment is followed by an IDI question about why they feel that way.

A benefit of this technique is that questions can be further clarified to guarantee respondent understanding and accurate responses. It also permits researchers to investigate certain fields if required but it keeps a framework that allows for statistical analysis.

Once again, the sample sizes are relatively small, but it is deemed as reliable data and is highly valuable when each respondent is a key interest group. It produces statistical and narrative data that is measurable and comparable.

Or in summary, structured IDIs are a compromise between extracting precise information from an individual by using quantitative surveys while still having the depth and richness of individual interviewing.