Study methodologies: Case control
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.
Case Control Studies
In a case control study, individuals with a certain condition (cases) are
compared with individuals without the condition (controls) in terms of history
of previous exposures, thereby moving backward in time. Like cohort studies,
this is a rarely used methodology in animal sentinel research. A study of
leptospirosis compared a sample of sick dogs with controls who were not
noticeably sick, and found greater evidence of previous leptospirosis infection
in the sick dogs (Weekes, Everard et al. 1997). Hayes et al used service
records to compare historical toxin exposures between military dogs with
and without lymphoma (Hayes, Tarone et al. 1995).
Case control studies, where exposures are retrospectively assessed between
cases and control subject, provide some advantages over cross-sectional
studies in terms of time order. There are obvious difficulties in reconstructing
historical exposures for a wild animal, although tissue samples, otolith
analysis, and exposure records for an area etc. could provide some clues
to previous exposures. As in cross-sectional studies sampling on the basis
of outcome, case-control studies can examine several different risk factors
at once, although only one disease outcome at a time can be studied. This
allows the investigator to adjust for possible confounding variables, and
economically explore several hypotheses simultaneously. Candidate hypotheses
generated from such studies can then be subjected to experimental confirmation.
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