A comparison of participants with non-participants in a population-based epidemiologic study: the Melbourne Visual Impairment Project.

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Abstract
Purpose: Adequate participation in population-based studies is essential to ensure that the sample is representative of the population under investigation. Participants may differ from non-participants on important variables such as age, sex, socioeconomic status, and general health factors. The Melbourne Visual Impairment Project (Melbourne VIP) is a population-based study designed to increase understanding of the prevalence and severity of common ocular disorders affecting people 40 years of age and over. Aim: The aim of this study was to determine the potential for any non-response bias by comparing data from participants and non-participants of the Melbourne VIP. Methods: Specific demographic and general variables were compared between the two groups. The variables included age, sex, education level, and social status. The reason for non-attendance was also recorded. Results: A total of 3271 (83%) eligible residents from the 9 sample areas were screened; 46% males and 54% females. Language spoken at home was significantly associated with participation. Residents whose main language at home was not English were less likely to attend the screening centre. (OR: 0.60; CI: 0.44–0.81). The main reasons given for non-attendance by eligible residents were lack of interest (6%), too busy to attend (4%), personal illness (2%), and attend own eye specialist (2%). Conclusion: We believe these results will not impact significantly on the interpretation of gender and age-specific data from the Melbourne VIP.