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The Canary Database
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Study methodologies: Cohort

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.

Cohort Studies

In a cohort study, exposed and unexposed individuals are compared over time in a forward direction to determine whether an outcome occurs. This is in general appears to be a rare methodology for animal sentinel studies. In a study that seemed to cross the line between experimental and observational, researchers used cages to segregate several populations of fish living in different proximities to a wastewater treatment facility, then compared responses in each group (Mitz and Giesy 1985). A study of Japanese encephalitis virus followed a group of originally seronegative "sentinel pigs" to determine which individuals became infected (Detels, Cross et al. 1976).

The forward directional approach of a cohort study is considered to approximate an experimental design. As in experimental studies, only one risk factor at a time can be examined in this way, whereas a number of different outcomes can be assessed. There are obviously challenges to tracking individuals in animal population over time. However, use of enclosures, or marking of sedentary populations using capture recapture techniques have already been employed, and may be worthy of greater exploration.

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