CommentaryEstimating neighborhood health effects: the challenges of causal inference in a complex world
Introduction
Oakes raises a series of important questions on the validity of past work on neighborhood health effects and suggests directions the field should take (Oakes, 2003). Indeed the limited nature of the evidence linking neighborhoods to individual-level outcomes, and its many methodological problems, have been noted in health and other fields (Diez Roux, 2001; Duncan & Raudenbush, 1999; Furstenberg & Hughes, 1997; Macintyre, Ellaway, & Cummins, 2002; Sampson et al., 2002; Tienda, 1991). The complex methodological issues inherent in the estimation of causal neighborhood effects are nevertheless worth reiterating and elaborating on as Oakes does. I would also posit that many of the issues Oakes raises are common to epidemiology generally (and to observational studies in other fields) and are not necessarily specific to research on neighborhood effects (or to so-called “social epidemiology” as he sometimes implies). Perhaps one of the problems is that in research on neighborhoods and health (as in epidemiology generally) the word “effects” is often used loosely, leading to the impression that the associations reported are always valid and precise estimates of generalizable causal parameters, when of course they are not. I will begin with some general comments on points raised by Oakes when he describes “a causal model for neighborhood effects”. I will then summarize agreements and disagreements with the methodological obstacles Oakes notes, and with the approaches he proposes.
Section snippets
The selection or confounding issue
Undoubtedly, the selection issue (the fact that persons may be selected into neighborhoods based on individual attributes which are themselves related to health) is the key problem in observational studies of neighborhood effects. Epidemiologists have attempted to account for this by controlling for often numerous individual-level variables. One of the problems, as Oakes notes, is that when numerous covariates are included it is likely that sparse data will be found in many cross-tabulated
Building the “causal multilevel model for neighborhood effects”
Oakes proposes a series of steps to construct what he refers to as “a causal multilevel model for neighborhood effects”. The steps he proposes are logical, based on existing multilevel analysis literature, and have often been followed (either explicitly or implicitly) by neighborhood effects researchers. As in model building generally, there is no single way to build a “causal multilevel model”, and using a given approach does not guarantee that the estimates obtained will be valid estimates of
Social stratification confounds comparisons
Oakes correctly notes that if “there is (approximately) complete confounding between the background attributes of persons in a given neighborhood and (approximately) complete separation between background attributes of people in other neighborhoods” it will be impossible to identify neighborhood effects. But the extent to which individual and neighborhood characteristics are so strongly associated is an empirical question that can be examined in the data. Although neighborhood characteristics
Where does this leave observational studies of neighborhood effects?
Have contextual and multilevel studies of neighborhood health effects concluded that the observed associations are causal? No. Should the conditional measures of associations they have generated be taken literally as valid and definite measures of causal effects parameters? Of course not, as most rigorous studies of neighborhood effects note. But although still extremely crude, taken together with other data (plus many of our daily living experiences which are after all, an important source of
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