'Inaccurate' responses
The next two types of non-sampling error are caused by inaccurate information being obtained from the sampled individuals.
Instrument error
Instrument error usually results from poorly designed questions. Different wording of questions can lead to different answers being given by a respondent. The wording of the question may be such as to elicit some particular response (a leading question) or it may simply be carelessly worded so that it is misinterpreted by some respondents.
Interviewer error
Interviewer error occurs when some characteristic of the interviewer, such as age or sex, affects the way in which respondents answer questions.