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
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.
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.