Study methodologies: Experimental
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.
Researchers often study environmental toxins in the controlled setting of
the laboratory. The Canary Database looks for such studies that use
"non-traditional" animal species representative of free-ranging or domestic
populations. Sometimes it is difficult to determine whether a laboratory
experiment was using animals derived from a truly wild (or free ranging
domestic) population rather than one bred strictly for laboratory use.
Some experimental studies in the database ventured outside the laboratory
to test the effect of slightly less controlled exposures. One team maintained
fish in a tank connected with a polluted river compared to controls raised
in tap water (Schmidt-Posthaus, Bernet et al. 2001). Such "enclosure"
studies blur the distinction between a tightly controlled laboratory
experiment, and the uncontrolled exposure situation in an observational
cohort study. Infectious disease studies also use experimental designs,
though much less commonly, and usually in conjunction with another study
Due to the fact that many variables can be tightly controlled, experimental
studies can provide invaluable confirmation of an association between an
exposure and an outcome. Yet in the overall investigation of an environmental
hazard or an unexplained health outcome in a population, the experimental
design has limitations. Constructing a controlled environment in the
laboratory setting and maintaining a laboratory population of animals can
be expensive and time consuming. The laboratory setting may also fail to
replicate some of the important forces at work in the environment. For
logistic reasons, experimental doses may be higher and follow-up time
shorter. Artificial diets and limitations on activity may also make it
difficult to extrapolate experimental findings to the real world setting.
Perhaps most importantly, an experiment can only test a very limited set
of hypotheses. Therefore, experiments would seem most appropriate only
after a number of observational studies have consistently provided support
for a limited number of hypotheses.