Unfortunately, it is expensive, supervision of the interviewer is difficult, and interviewer bias may arise. The interview script must include questions read aloud, and instructions that reduce the chance of the interviewer making a mistake. Mailed surveys can effectively elicit information. Respondents feel less intimidated about giving information because they are not talking to another person. Mailed surveys are researcher-friendly because of the low cost per questionnaire.
Weaknesses are low response rates, lack of opportunity to probe, and a long timeline.
The telephone survey is the most common survey method. Unfortunately, minorities and males are underrepresented in responding households. A telephone survey is a quick means of collecting data, supervision of interviewers is easy, and response rates are good. This may result from too many open-ended questions, badly worded questions, or respondents without enough information to give an opinion. You can head off these difficulties by pre-testing your survey.