Navigating non-positivity in neighbourhood studies: an analysis of collective efficacy and violence
- Jennifer Ahern1,
- Magdalena Cerdá2,
- Sheri A Lippman3,
- Kenneth J Tardiff4,
- David Vlahov5,
- Sandro Galea2
- 1Division of Epidemiology, Berkeley School of Public Health, University of California, Berkeley, California, USA
- 2Department of Epidemiology, Columbia University Mailman School of Public Health, New York, New York, USA
- 3Center for AIDS Prevention Studies, Department of Medicine, University of California, San Francisco, California, USA
- 4Cornell University Weill School of Medicine, New York, New York, USA
- 5School of Nursing, University of California, San Francisco, California, USA
- Correspondence to Dr Jennifer Ahern, School of Public Health, University of California, Berkeley, 101 Haviland Hall, Berkeley, CA 94720-7358, USA;
Contributors JA collaborated on the design and implementation of the study, designed and implemented the analysis, conducted the literature review and wrote the manuscript. MC collaborated on the analysis plan, drafted sections of the manuscript and substantially edited all sections of the manuscript. SAL collaborated on the literature review, provided input on the analysis plan and substantially edited all sections of the manuscript. KJT obtained study funding and provided input on the analysis plan and on the manuscript. DV obtained study funding and provided input on the analysis plan and on the manuscript. SG obtained study funding, collaborated on the design and implementation of the study, and substantially edited all sections of the manuscript. All authors have approved the final version of the manuscript.
- Accepted 23 July 2012
- Published Online First 22 August 2012
Background In multilevel studies, strong correlations of neighbourhood exposures with individual and neighbourhood confounders may generate problems with non-positivity (ie, inferences that are ‘off-support’). The authors used propensity restriction and matching to (1) assess the utility of propensity restriction to ensure analyses are ‘on-support’ and (2) examine the relation between collective efficacy and violence in a previously unstudied city.
Methods Associations between neighbourhood collective efficacy and violent victimisation were estimated in data from New York City in 2005 (n=4000) using marginal models and propensity matching.
Results In marginal models adjusted for individual confounders and limited to observations ‘on-support’, under conditions of high collective efficacy, the estimated prevalence of violent victimisation was 3.5/100, while under conditions of low collective efficacy, it was 7.5/100, resulting in a difference of 4.0/100 (95% CI 2.6 to 5.8). In propensity-matched analysis, the comparable difference was 4.0/100 (95% CI 2.1 to 5.9). In analyses adjusted for individual and neighbourhood confounders and limited to observations ’on-support’, the difference in violent victimisation associated with collective efficacy was 3.1/100 (95% CI 1.2 to 5.2) in marginal models and 2.4/100 (95% CI 0.2 to 4.5) in propensity-matched analysis. Analyses without support restrictions produced surprisingly similar results.
Conclusions Under conditions of high collective efficacy, there was about half the prevalence of violence compared with low collective efficacy. The results contribute to a growing body of evidence that suggests collective efficacy may shape violence, and illustrate how careful techniques can be used to disentangle exposures from highly correlated confounders without relying on model extrapolation.
Funding Support for this work was provided in part by the National Institute on Drug Abuse at the National Institutes of Health (DA 017642 and DA 06534).
Competing interests None.
Ethics approval The study protocol was approved by the institutional review boards of the New York Academy of Medicine, the University of Michigan and the University of California, Berkeley.
Provenance and peer review Not commissioned; externally peer reviewed.