Due to the existence of structural confounding some empty
Due to the existence of structural confounding, some empty MTT were observed in our stratified table. Since these empty cells represent a deterministic non-positivity they cannot be filled by more data collection, as opposed to random non-positivity (Westreich & Cole, 2010). Our findings are consistent with those of a study conducted by Messer et al. (2010) in the socially stratified and racially segregated context of neighbourhoods of North Carolina where no white woman lived in the most deprived areas and no black woman lived in privileged neighbourhoods; however, this contrasts with our previous study of Canadian adolescents. Including the same three community level variables of SES, built environment, and social capital and utilising a similar methodology, there was no sign of structural confounding in a nationally representative sample of over 26,000 Canadian students aged 11–15 years (Vafaiei et al., 2014). The discrepancy between these child findings and the current study of older adults may relate to the fact that older adult communities are more directly influenced by social factors, therefore living in poor neighbourhoods also negatively impacts levels of social interactions. Using neighbourhoods defined by administrative boundaries versus the school as used in the adolescents study is another potential reason, since compared to an educational context, place of residence may impose a more direct social stratification. Finally, existence of structural confounding precludes causal inference in regression analyses due to scant data and should be taken into account in etiological aging research.
Our study also has some limitations. Our participants were not a representative sample of the Canadian population of older adults and the results are therefore not generalisable to that population. The Kingston sample was dominated by very highly educated individuals who might interpret social capital questions differently, therefore bias attributable to this selection issue is likely. The direction of this bias remains to be explored in future studies. It is also worth noting that in choosing aggregate level data we were limited to the predefined administrative boundaries provided by Statistics Canada for the SES variables, and municipal boundaries provided by City of Kingston and Ville de Saint-Hyacinthe for other variables. Despite significant differences in individual and neighbourhood levels of SES across the two study sites (Table 1), indicators of physical and mental health, mobility, grip strength as well as BMI, smoking status, and levels of most laboratory indicators showed similar distributions in Kingston and Saint-Hyacinthe (Appendix Table 2). This suggests that the two sample populations were sufficiently homogeneous to be appropriately combinable.
Same-source (Diez-Roux, 2007) or mono-method bias is also a potential threat to the validity of Microsomes study. This bias happens when self-reported data are used for both the outcome and the neighbourhood characteristics, and generates a spurious association because of the correlation between measurement errors in both exposure and outcome or because the outcome affects the perception of the neighbourhood attribute. This is not a major methodological concern in our circumstances. We aggregated our measure to neighbourhoods and there are some studies that conclude that aggregation of self-perceived measures to narrow area units reduces the possibility of this type of bias (de Jong et al., 2011). Also, this bias is more applicable to outcomes measured purely by individuals\' subjective perception such as depression and level of physical activity. In older adults, a fall is an occurrence that is unlikely to be forgotten or over-reported because of other factors.
Study strengths and limitations The study from which these findings derive sought to focus on children\'s own views, which have been relatively underexplored in the context of health promotion. The positive and productive research relationships forged during the study allowed the generation of in-depth data. Opportunities afforded by the research design for children to elaborate on and reconsider their ideas by working with them on two and sometimes three occasions, the inclusion of task-based activities and children\'s ability to work in friendship groups in schools all helped to foster a positive environment in which children could reflect upon and share their ideas. Nevertheless, there are inevitable limitations to such a study that need to be acknowledged. Fifty-three children participated and the study, only four of whom were from minority ethnic groups and no claim for empirical generalizability can therefore be made. Furthermore, following Mason (2002) we acknowledge that explanations do not simply emerge from the data and different interpretations could be crafted by applying different theoretical lenses (p.149). Nevertheless, by carefully articulating the study context and by seeking to lay over interpretations onto data \'without doing violence to them\' (Richards and Richards, 1994), the findings presented in this paper accord with Mason\'s notion of theoretical generalisability in which the \'detailed and holistic explanation of one setting, or set of processes, [can] frame relevant questions about others\' (Mason, 2002, p. 196).