Interviewer bias - interviewers can systematically influence or distort the results of oral surveys; this can happen unconsciously (solely through appearance, appearance, way of speaking and recording behavior) or consciously (through the selection of the respondents, improper question formulation, advice on answering or even falsifying questionnaires). Because of the multitude of factors that influence the behavior of the interviewer and respondent during the survey (age, education, gender, personality traits, observable behavior, etc.), it is often difficult to completely avoid or eliminate these falsifications.
To reduce the influence of the interviewer, extensive standardization of the questionnaire and careful selection, training and specific instruction of the interviewer is recommended. The following control measures can be taken to prevent falsified interviews: use of bogus control addresses, random checks through follow-up phone calls or visits, statistical checks for systematic deviations in the results.