Where CATI surveys follow a more rigid structure, depth surveys are carried out between a consultant and a respondent in a more open ended fashion. For this type of survey the consultant is knowledgeable about the main goals and subjects of the talk, but has a limited number of specific questions defined prior to the interview. This demands a lot more from the consultant than more closed-end surveys and will often take up to 45 to 120 minutes to carry out.
Depth surveys ensure that we pick up information on areas that we may not have thought were important prior to the interview and can often be one of the first steps in building up a suitable closed-end questionnaire for a larger survey!