Objective: To investigate whether interviewer personality, sex or being of the same sex as the interviewee, and training account for variance between interviewers’ ratings in a medical student selection interview. Design, setting and participants: In 2006 and 2007, data were collected from cohorts of each year's interviewers (by survey) and interviewees (by interview) participating in a multiple mini-interview (MMI) process to select students for an undergraduate medical degree in Australia. MMI scores were analysed and, to account for the nested nature of the data, multilevel modelling was used. Main outcome measures: Interviewer ratings; variance in interviewee scores. Results: In 2006, 153 interviewers (94% response rate) and 268 interviewees (78%) participated in the study. In 2007, 139 interviewers (86%) and 238 interviewees (74%) participated. Interviewers with high levels of agreeableness gave higher interview ratings (correlation coefficient [r] = 0.26 in 2006; r = 0.24 in 2007) and, in 2007, those with high levels of neuroticism gave lower ratings (r = − 0.25). In 2006 but not 2007, female interviewers gave higher overall ratings to male and female interviewees (t = 2.99, P = 0.003 in 2006; t = 2.16, P = 0.03 in 2007) but interviewer and interviewee being of the same sex did not affect ratings in either year. The amount of variance in interviewee scores attributable to differences between interviewers ranged from 3.1% to 24.8%, with the mean variance reducing after skills-based training (20.2% to 7.0%; t = 4.42, P = 0.004). Conclusion: This study indicates that rating leniency is associated with personality and sex of interviewers, but the effect is small. Random allocation of interviewers, similar proportions of male and female interviewers across applicant interview groups, use of the MMI format, and skills-based interviewer training are all likely to reduce the effect of variance between interviewers.