These interviews are a useful market research method - particularly for interviewing hard to reach or elderly individuals or for gaining qualitative insights into emotional or sensitive topics.
Whilst these are usually carried out face-to-face, they can also be carried out by telephone - particularly useful for conducting interviews with geographically dispersed people and time-pressed individuals such as business owners and senior decision makers.
Depth interviews are typically conducted on a one-to-one basis (interviewer and respondent) although there may be occasions where "paired" depths may be recommended for example, under 16 year olds where friends are interviewed together and where a parent would need to be present.
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added January 19th 2018
added October 20th 2017