When collecting data for a study, researchers often choose to use questionnaires because they are cost-effective, time-efficient and easy to evaluate objectively. In spite of these benefits, questionnaires have many deficiencies. As such, researchers often choose to use interviewers over questionnaires, as personal interviews can provide more information about the test subject's answers while providing the same sort of statistical precision.
Interviews can be more useful than questionnaires because they allow researchers to collect non-verbal data. For example, researchers can see whether particular questions make an interview subject nervous or whether the test subject struggles to answer the question. In short, nonverbal cues such as lack of eye contact, jittery mannerisms or defensive posturing can provide context to an interviewee's answers. This type of information could not be collected from a written questionnaire.
Because research subjects typically complete questionnaires without the assistance of a researcher or test proctor, it is difficult to know whether the interviewee understands the questions she is being asked. When a researcher conducts a live interview, however, the test subject can ask for clarification if she does not understand a question. Likewise, the interviewer can ask follow-up questions to evoke a more thorough response. Ultimately, this leads to more detailed and thorough data.
Written questionnaires are not a viable choice for researchers who wish to study young children, people who are illiterate or individuals who are visually impaired. A researcher can eliminate these logistical limitations by conducting an interview. Interviews lower the test subject's anxiety level, making it well suited for sensitive subject matters.
Professor Mathieu Deflem of the University of South Carolina explains that interviews are more flexible than questionnaires and are a good fit for studies where the research question is not well defined. Because the "interviewer is the central instrument of investigation," he can bring up new issues that may be relevant to the study which arise during the course of the discussion with the test subject. As such, interviews are a stronger tool than questionnaires for researchers who want to explore topics in a general way.