The interviewer in a research study can influence the outcome by the type of information that is collected. This is leads to type of non-random misclassification bias.
Examples of interviewer bias:
(1) the interviewer asks different questions of a subject depending on whether the subject is a patient or control
(2) the interviewer influences the responses that the subjects give
(3) the interviewer is more or less diligent than the other interviewers
(4) the interviewer is preferentially assigned to one group over another
Methods to minimize interviewer bias:
(1) blinding of the interviewer
(2) training the interviewer to avoid variation in the interview process
(3) standardizing the interview as much as possible so that there is little opportunity for variation
(4) supervising and observing the interviewer
(5) random assignment of the interviewer
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