Comparison of the sociodemographic characteristics of the large NutriNet-Sante e-cohort with French Census data: the issue of volunteer bias revisited

Background A recurring concern in traditional and in Web-based studies pertains to non-representativeness due to volunteer bias. We investigated this issue in an ongoing, large population-based e-cohort. Methods The sample included 122 912 individuals enrolled in the Internet-based, nutrition-focused NutriNet-Sante study between May 2009 and March 2014, with complete baseline data. Participants were recruited via recurrent multimedia campaigns and other traditional and online strategies. Individuals aged 18+ years, residing in France and having Internet access, were eligible for enrolment. Their sociodemographic characteristics were compared with the corresponding 2009 Census data via chi(2) goodness-of-fit tests. The effectiveness of statistical weighting of the e-cohort data was also explored. Results The sample exhibited marked geographical and sociodemographic diversity, including volunteers belonging to typically under-represented subgroups in traditional surveys (unemployed, immigrants, the elderly). Nonetheless, the proportions of women, relatively well-educated individuals and those who are married or cohabiting, were notably larger compared with the corresponding national figures (women: 78.0% vs 52.4%; postsecondary education: 61.5% vs 24.9%; married or cohabiting: 70.8% vs 62.0%, respectively; all p< 0.0001). Conclusions There were notable sociodemographic differences between the general French population and this general population-based e-cohort, some of which were corrected by statistical weighting. The findings bear on the potential generalisability of future investigations in the context of e-epidemiology.