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

population and that the controls were selected at the same time as the cases from among women in
that population (this is called "density sampling of controls").
Estrogen
Endometrial
cancer
Yes No Total
Case a b m1
(a + b)
Control c d m0 (c + d)
Total n1 n0 n
(a + c) (b + d)
If this situation, the cases would be all (or some fraction f1 of) cases of endometrial cancer in the
population. If the incidence rate of endometrial cancer is ID and the amount of population-time is
N women-years, then:
m1 = (f1)(ID)(N)
[f1 is included only for purposes of generality — if all cases are included, then f1=1 and can be
ignored.]
Cases among women taking estrogen (cell "a") would be:
a = (f1)(ID1)(N1)
where ID1 and N1 are the incidence rate and population-time, respectively, for women taking
estrogen.
Similarly, cases among women not taking estrogen (cell "b") would be:
b = (f1)(ID0)(N0)
with ID0 and N0 applying to women not taking estrogen.
Note: Whether N1 and N0 represent women-years of estrogen use or women-years in estrogen
users (i.e., are person-years for a women after she stops taking estrogen counted as exposed or
unexposed) would depend upon whether the estrogen effect endures after the drug is discontinued.
We now see where the cases have come from. What about the controls? The control group is
typically, though not necessarily, chosen in some fixed ratio to the number of cases, such as two
controls per case.
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www.sph.unc.edu/courses/EPID168, © Victor J. Schoenbach 1999 8. Analytic study designs - 236
rev. 9/6/1999, 10/7/1999, 12/17/1999