This chapter explores the effect that interviewers have on the variance of survey estimates. It describes in more detail the ways in which interviewers are thought to contribute to the survey design effect. This is followed by a description of the data, the measures to be used and the analysis strategy. The term 'design effect' (DEFF) refers to the part of the variance of survey estimates which arises as a result of complex design features such as stratification and clustering. The chapter reveals the extent to which the interviewer variance component differs across a range of more psychologically oriented measures of interviewer characteristics, including survey experience, satisfaction with pay, attitudes toward interviewing, self-rated communication skills and two dimensions of the Big Five personality inventory. The National Travel Survey (NTS) questions focus on rather mundane matters of transport infrastructure and may not be particularly susceptible to personality-related interviewer effects while questions of a more personal and sensitive nature are.