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The Canary Database
Center for One Health Research
University of Washington

Study methodologies: Aggregate

The Canary database curators determine, for each included study, the type of study methodology employed by the researchers (using this classification protocol). The possible categories are:

Fox has outlined criteria for objectively evaluating the relationship between an environmental hazard and an observed health effect in an observational study of animals (Fox 1991). These include probability, time order, strength of association, specificity, and consistency on replication, predictive performance, and coherence. The choice of study design can have a major effect on the ability of a study to fulfill such criteria.

Our preliminary review of the animal sentinel literature has found that some potentially useful study designs, such as case-control and cohort, are under-utilized in animal sentinel research.

Aggregate (Ecologic) Studies

In aggregate (also known as ecologic) studies there are comparison between groups, but exposures were not determined on an individual level. This is a common study design for toxin studies. An example is a study of the effect of sewage on sex hormone levels in fish. Sex steroid levels for river fish living upstream and downstream from a sewage treatment plant were compared. The researchers found depressed testosterone in male fish living below the sewage outflow. However, there was no attempt to measure levels of contaminants in individual fish (Folmar, Denslow et al. 2001). Some aggregate studies were repeated over time, as when pooled samples of organochlorines in eggs and pooled assays of immune function were compared by year for groups of terns in a polluted and unpolluted site (Grasman and Fox 2001).

These aggregate studies, where rates of disease are compared to other factors in different geographic areas, are fairly simple to undertake and have advantages over case reports for examining hypotheses, since comparisons are made between groups with presumably different exposures, yet again they should not be used to draw causal conclusions. Since exposures are not assessed on the level of the individual, there is no way of knowing if the diseased individuals were actually exposed. For example, in the study of terns and organochlorines, the pooling of samples made it impossible to say that a more affected tern also had a higher level of contaminant. Such studies therefore run the risk of the "ecological fallacy" where an association is postulated that may actually not be true, as in the assertion that high levels of a certain contaminant in the environment and high levels of a particular animal deformity mean that the contaminant is responsible for the deformity.

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