More structured the interviews; the more likely they are to be used in a quantitative way to produce statistics.
More unstructured the interviews, the more likely they are to be used in a qualitative way (used by interpretive sociologists).
Issues of Validity and Reliability.
Validity: the type of questions asked in the questionnaire/interview must allow the respondent to give a true and accurate reply.
Reliability: if questions mean different things to people or can be interpreted differently, then research is not reliable.