Question wording . The wording of a question can influence the response enormously. Sensitive topics . It is hard to get honest answers to questions on sensitive topics, including income, sex, hygiene, and nefarious or illegal behavior, such as tax evasion or cheating on exams. Interviewer bias .
Interviewers can bias the results consciously or unconsciously, in a variety of ways.
For example, if the interviewer has any discretion in selecting subjects to interview, the sample is likely to be biased towards persons the interviewer finds approachable. The appearance and demeanor of the interviewer affect people's willingness to respond. Also, most subjects unconsciously seek to please the interviewer. The apearance and demeanor of the interviewer influence what subjects imagine the interviewer to be thinking. A man with a military haircut wearing a business suit asking whether there should be more homeless shelters probably would get different responses than would a shabbily dressed woman with an infant in her arms.