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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