Fundamentals of epidemiology - an evolving text - Are you looking ...

Validity
The validity of a case-control study requires that:
Cases in the study adequately represent the relevant cases (the population of cases about whom
inferences are to be made) with respect to the variables of interest (notably, prevalence of
exposure). This depends upon whether the cases available do in fact reflect the rates of disease
in exposed and unexposed individuals undistorted by differential manifestation, detection, or
short-term survival (e.g., selective survival, access to care, detection bias);
Controls accurately reflect the exposure proportions in the study base (the source population for
the cases). For example, hospitalized controls may overrepresent exposures associated with
hospitalization for other conditions.
Both of these requirements, especially the latter, can be difficult to ensure. Therefore, case-control
studies are regarded as highly susceptible to bias from problems with the:
Identification of cases
Reliance on medical care system
Often miss subclinical cases (detection bias?)
Can miss rapidly fatal cases (selectively?)
Selection of controls
Selection of controls can determine the study results
Which controls are appropriate is often not obvious
Trade-off between sampling and data collection
Hospitalized controls, community controls, dead controls
Controls may be reluctant to cooperate
Measurement of exposure for cases and controls
Reliance on recall or records (differential?)
Effect of disease on exposure assessment
Effect of disease on exposure (confounding by indication)
There is also the thorny problem of establishing temporality, i.e., did the exposure precede the
disease?
Interpretability of the odds ratio
Why does the OR from the cases and controls we have assembled estimate anything in the
population? Consider what the cells in the table below represent. Assume that the cases were
selected as newly occurring cases of endometrial cancer over a period of time in a defined
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www.sph.unc.edu/courses/EPID168, © Victor J. Schoenbach 1999 8. Analytic study designs - 235
rev. 9/6/1999, 10/7/1999, 12/17/1999