The Oslo Health Study: Reminding the non-responders - effects on prevalence estimates

Background: Low response rates in population studies may lead to serious selection bias. Most surveys try to increase attendance by sending one or more reminders, but we do not know whether these efforts actually lead to less bias. In the Oslo Health Study we have previously shown that sociodemographic background variables are distributed differently in attendees and non-attendees. The aim of this study is to investigate the effect of sending reminders to increase the rates of attendance in the Oslo Health Study, on the distribution of demographic variables and on the prevalence estimates of selected variables. Methods: The analyses were based on linkage between data from public registers in Statistics Norway and data from the Oslo Health Study, a cross sectional population-based survey inviting all citizens in five agegroups 30-76 years old. Demographic variables from public health registers were compared in persons attending directly without reminders (n=12,495), in all participants attending after up to two reminders (n=18,769) and in the total invited population (n=40,874). Prevalence estimates were compared in the two groups of participants attending directly (n=12,495) and after one or two reminders (n=6,274) and in the combined group of all participants. Results: By means of two reminders the attendance rate increased from 28% to 42% in men and from 33% to 49% in women. When comparing participants attending directly with the sample including all participants and the total invited population, the percentage in the 30 year olds increased from 20% to 22% and 28%. The sample included successively more unmarried persons, 28% and 29% versus 35% in the total population, more persons born in non-western countries, 9%, 10% and 12%, respectively, citizens living in the inner east district, 13%, 14% and 16%, with lower secondary education, 14%, 15% and 17% and persons in the lowest income category, 9%, 10% and 12%. When comparing the sample attending directly with the sample including all participants, the prevalence of diabetes in age group 75-76 years, increased from 8 to 9% in men and from 4 to 6% in women. Similarly, there was an increase in percent daily smokers, 14% vs. 16% in men and 16% vs. 17% in women, and symptoms of anxiety/depression (HSCL), 4% vs. 5% in men and 11% vs. 12% in women. The prevalence of good self-reported health decreased (70% vs. 67% in men, 60% vs. 57% in women) in 75-76 year olds. There was no change in the other age groups. Persons born in non-western countries returned the first supplementary questionnaire (57% of those attending) to a lesser extent than did persons born in Norway (87%). Conclusions: When including participants attending after reminders the sociodemographic distribution moved somewhat towards the distribution in the target population. The estimated prevalence figures showed small changes in the oldest age group and almost no change in the other groups. Increasing attendance through reminders had only minor effects on prevalence estimates and conclusions.