Ecological Bias
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All empirical studies face difficulties with making causal inferences, but
ecological studies face special sources of bias in dealing with exposures and
confounders. These difficulties arise because of the aggregation of observations
and because the data on exposures, confounders, and outcomes are
from different sources. At the most basic level, the data on firearms ownership
in these studies may not come from the persons who committed suicide.
Thus, ecological studies cannot establish whether there is a relation
between gun ownership by an individual or household and suicide by that
individual or member of the household. This may seem like a small problem
in the case of gun suicide; after all, the victims of a gun suicide have
undeniably achieved access to a gun. But community-level rates of gun
ownership may not reflect the rates of gun ownership among highly suicidal
persons. If, for example, the relationship between gun access and gun suicide
varies by age and sex or by psychiatric disorder, then the aggregate
association may reflect differences in the prevalence of suicidal states among
persons of different age and sex or psychiatric disorder in the population,
rather than differences in access to firearms. The geographical level of
aggregation in state-level or regional ecological studies may be so high that
there is no way of knowing whether the gun homicides or gun suicides
occurred in the same areas with high levels of gun ownership.
Thus, even if FS/S is found to be a valid proxy for state-level gun
prevalence, something that is not yet established, ecological studies may
lead to biased inferences. The proxy is not a substitute for good data on
household-level ownership or even ownership at a smaller level of aggregation
by age, sex, or geography. Rather, better individual-level studies exploring
the relationship between gun ownership and suicide may be needed
in order to further understanding of the overall relationship between firearms
and the risk of suicide.