Practical causal inference for ecoepidemiologists

Environmental scientists and managers must determine whether a relationship between an environmental factor and an observed effect is causal and respond accordingly. Epidemiologists have, over the past 150 yr, developed a systematic approach to evaluating these relationships. Their criteria for objectively evaluating the relationship between a suspect cause and a chronic disease are (1) probability, (2) time order, (3) strength of association, (4) specificity, (5) consistency on replication, (6) predictive performance, and (7) coherence. These criteria can be used, with little modification, to evaluate associations in relation to diseases in fish and wildlife suspected to be caused by exposure to chemical pollutants. Some populations of fish and wildlife are members of the same guilds as subpopulations of humans. Investigations of chemically induced disease in these sentinel populations of fish and wildlife may identify the potential risks posed to these human subpopulations. Evidence evaluated using the epidemiologic criteria may assist environmental managers to determine whether a substantive case can be made to initiate preventative or remedial action. By applying the null hypothesis, scientists are forced to consider how much information must be ignored to conclude that a causal relationship does not exist.