The fact that psychiatric questions were not the primary objectives of HUNT made interest-related bias less likely. Some research has indicated that people with psychiatric disorders are over-represented among non-responders in studies of the general population [28]. This could be a weakness, since 30% of invited individuals did not attend [16]. But according to a study on non-attendants, health related reasons for not attending were unlikely among individuals below 70 years of age [17]. Our sample of more than 4000 individuals should have sufficient power to detect a difference of clinical interest in prevalence of depression and anxiety between genotypes. Because of the large sample size, we had enough power to present separate analyses by gender.
