PhD thesis - University of Hertfordshire Research Archive

they would for other bacterial gastrointestinal pathogens (Rooney et al.,
2000).
4.2 Case-control studies
Case-control studies are observational studies used frequently in infectious
disease epidemiology. They are relatively simple to plan and execute, and
are particularly useful for rare diseases or those where other epidemiological
approaches would be prohibitively expensive (e.g. cohort studies) or
unethical (e.g. randomised control trials to investigate the potentially toxic
effects of chemicals). Fundamental to the case-control study is the
establishment of an outcome of interest (e.g. confirmed infection, hospital
admission, death etc) which distinguishes „cases‟ from „controls‟ within a
particular study population. Exposure information leading up to this outcome
is then sought (either retrospectively or prospectively) for both cases and
controls and appropriate statistical comparisons of these data are undertaken
to identify factors particular to cases i.e. „risk factors‟ for the outcome of
interest.
Case-control studies are, however, subject to a number of biases which limit
their effectiveness and have the potential to distort their findings. Firstly,
because they are identified through surveillance, cases usually include
laboratory-confirmed infections who are selected non-randomly. Infants,
children and people with more severe/prolonged disease are more likely to
present to and be seen by a primary care physician, and they may be more
likely to submit a sample for laboratory testing. Furthermore, laboratory-
confirmed cases might be more willing to participate in a study depending on
their exposures, outcomes or both. For example, patients who experienced a
more severe illness might be more inclined to participate in a study than
patients with milder infections, or they might be more assiduous in their
responses to study questions. Alternatively, patients who believe that they
contracted their illness at a restaurant might be more willing to participate if
they think that it might facilitate compensation claims against the
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