Please identify a quantitative research article evaluating mediation in your field and provide the citation.
Nandi A, Galea S, Ahern J, Bucciarelli A, Vlahov D, Tardiff K. What explains the association between neighborhood-level income inequality and the risk of fatal overdose in New York City? Soc Sc & Med. 2006 (63);662-674.
What is the primary discipline of the authors?
Arijit Nandi: Dept of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, and Center for Urban Epidemiologic Studies, The New York Academy of Medicine, New York, NY
Sandro Galea: Center for Urban Epidemiologic Studies, The New York Academy of Medicine, New York, NY, and Dept of Epi, U Michigan School of Public Health, Ann Arbor MI
Jennifer Ahern: Center for Urban Epidemiologic Studies, The New York Academy of Medicine, New York, NY, and Dept of Epi, U Michigan School of Public Health, Ann Arbor MI, and Dept of Epi, UC Berkeley School of Pub Health, Berkeley CA
Angela Bucciarelli: Center for Urban Epidemiologic Studies, The New York Academy of Medicine, New York, NY
David Vlahov: Dept of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, and Center for Urban Epidemiologic Studies, The New York Academy of Medicine, New York, NY
Kenneth Tardiff: Dept of Psychiatry, Weill Medical College of Cornell University
What is the exposure of interest?
Neighborhood-level income inequality
What is the outcome of interest?
Overdose mortality
What is the hypothesized mediator of interest and how is it measured?
This study hypothesized three different factors as potential mediators between neighborhood-level income inequality and overdose death.
- Environmental disorder
- Level of police activity
- Quality of built environment
The authors used data from the NYC Mayor’s Management Report, the NYC Police Department, and the NYC Housing and Vacancy Survey to define constructs for the level of environmental disorder, police activity and quality of the built environment.
Describe the modeling approach and briefly report the estimated total, direct, and indirect effects (if these are reported).
The authors used multivariable models (with individual-level and neighborhood-level covariates and an individual-level outcome) to measure the odds of death due to drug overdose in neighborhoods in the top decile of income inequality compared to the most equitable neighborhoods, and adjusted for the three potential mediators. The authors also measured this potential mediation using path analysis, which was conducted at the neighborhood-level. Thirty-six percent of the association between the distribution of income and the rate of overdose death is explained by the direct effect of income inequality on the rate of drug overdose death (direct effect=0.21). Sixty-four percent of the association is indirect and mediated by levels of environmental disorder, levels of police activity and the quality of the built environment (indirect effect=(-0.55x-0.40)+(0.34x0.09)+(0.36x0.36)=0.38). After adjusting for the three mediators, the odds of overdose death in neighborhoods in the top decile of income inequality decreased from 163 to 1.23 when compared to the most equitable neighborhoods. The authors concluded that the association between income inequality and the rate of drug overdose mortality was primarily explained by an indirect effect through the level of environmental disorder and the quality of the built environment in a neighborhood.
Do you think there is potential measurement error in the mediator and how would that affect the results?
One potential measurement error relates to the way the authors chose to measure their mediators (they point this out in their limitations section). “Quality of the built environment” was based on the percent of housing units in dilapidated conditions which was based off of an assessment of occupied housing units only. This is likely to underestimate the mediating effect of the “built environment” variable on the relationship between overdose mortality and income inequality. Another concern I have is that the data sources the authors used were not collected at the same time. While neighborhoods change at a fairly slow pace, I still think it’s problematic to assess neighborhood-level characteristics in the same analysis as income inequality and overdose mortality if they are captured at different time points.
Do you think there are unmeasured confounders of the mediator-outcome association and how would that affect the results of the mediation analysis?
One potential confounder that was not included in the analysis was individual-level income (the study only uses neighborhood-level SES). We cannot assume that individual income levels are the same among all inhabitants of a specific neighborhood and it could have an important effect on both the exposure and outcome here. Another potential confounder is the type of drug involved in the fatal overdose. The authors do not account at all for the difference in drug types e.g. heroin versus prescribed opioids but I believe drug type or drug of choice could be a potential confounder in this analysis.
Do you have any critiques of the paper?
The mediators considered in this analysis are difficult to measure. The authors used reasonable proxies to measure quality of the built environment, police activity and environmental disorder. However, that may not necessarily represent the perception of these mediators from the neighborhood residents (see measurement error above). Police activity, for instance, was measured by the total number of recorded misdemeanors in the neighborhood. This, however, does not necessarily represent the total amount or type (e.g. aggressive, passive) of police activity within a neighborhood. Finally, I am concerned that the measure of overdose mortality was measured by the location of the death. Individuals at risk for overdose may be at risk of overdosing in neighborhoods where they buy drugs (which could be more likely to be neighborhoods with higher income inequality) however this may not necessarily represent their neighborhood residence. Finally, the outcome relies on medical examiner data but we know that it is difficult to capture all overdose deaths by toxicology reports and it is likely that a substantial number of overdose deaths were not necessarily captures in the medical examiner reports.