Abstract. If a survey is executed face to face by an interviewer, interviewer’s existence will have a great effect on the surveys costs and also data quality. Interviewer error occurs due to interviewers’ inaccuracy which leads to alterations in response.

In this paper besides reviewing non-sampling errors, a simulation is performed and variance if the mean is estimated for simulated data which are under the influence of interviewer error.

Also variance of estimator in an imaginary survey with applying three groups of interviewers are calculated and compared; meanwhile interviewer workload effect on estimators precision are compared too. In the end, achieved results regarding to the interviewer error are compared with classic formulas (regardless of interviewer error). Results of the simulation show that when interviewer workload increases the precision of the estimations decrease.