Ecological Bias

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.