This is where respondents are free to choose how to answer the question, i.e., they don't select 'yes' or 'no' or provide a numeric rating, etc.

An interview is a live one-on-one discussion with another person where the interviewer asks the interviewee a series of questions.  Interviews are particularly useful for getting the story behind a participant's experiences.  The interviewer can pursue in-depth information around a topic.  Interviews may be useful as follow-up to certain respondents to questionnaires, e.g., to further investigate their responses.