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To understand further the design of a case–control study,
and why the odds ratio from a case–control study estimates
the incidence rate ratio, consider first the occurrence of a particular
disease within an underlying population that gives rise to the cases.
This underlying cohort is sometimes called the source population.
At the start of follow-up, all subjects are disease-free; P people are exposed and Q are not. After following the cohort
for t years, A of
the P exposed and B of
the Q unexposed subjects develop disease
(Table C–1).
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To calculate incidence rates for this cohort, as discussed in
Chapter 2: Epidemiologic Measures, first calculate the person-years of observation (py), which is given by
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This equation simplifies if few people develop disease during
the follow-up period and the population undergoes no major demographic
shifts, a situation termed the “steady
state.” If the steady state holds, the size of the
source population is nearly constant and the equation above simplifies
to
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Application of the latter equation to the hypothetical cohort
yields P × t and Q × t person-years
of observation for the exposed and unexposed subjects, respectively
(Table C–1). Thus, the incidence rate in the exposed subjects
is A/(P × t) and in the unexposed subjects is B/(Q × t), and the incidence rate ratio (IRR) is
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Now, turning to the case–control study design, cases
arise from a clearly defined source population and the investigator
then chooses controls from this same population. Thus, to conduct
a case–control study using the source population described
in Table C–1, subjects with disease (cases) and subjects
without disease (controls) are sampled and their respective exposure
histories are then determined. Excluded from both the case and control
groups are potential subjects known to have had disease when the
study began—only newly diagnosed or “incident” cases are included.
In practice, it is possible to contact newly diagnosed cases and
a sample from the general population, ask each subject about prior
disease, and exclude those who were diagnosed prior to the study
period. The data can be summarized as in Table C–2.
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