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

The study population is a subset of the actual population. Bias is the discrepancy between the actual
and target populations. Generalizability deals with inference from the target population to an
external population (see previous page).
In thinking about selection bias and its potential effect on study results, we find it useful to consider
the probabilities according to which people in the target population could gain access to the actual
population. These probabilities are called (population) selection probabilities.
For simplicity, consider a dichotomous disease and dichotomous exposure classification, and let the
fourfold table in the target population and actual population be as follows:
_ _
E E E E
D A B D A o B o
_ _
D C D D C o D o
We can then define four selection probabilities:
Target Actual
alpha (α) = (A o /A) the probability that a person in cell A (in the target population) will be
selected into the actual population from which the study population is a random sample
beta (β) = (B o /B) the probability that a person in cell B (in the target population) will be
selected into the actual population
gamma (γ) = (C o /C) the probability that a person in cell C (in the target population) will be
selected into the actual population
delta (δ) = (D o /D) the probability that a person in cell D (in the target population) will be
selected into the actual population
Example: assume that selective survival exists, such that cigarette smokers who suffer an MI are
more likely to die before reaching the hospital. Then a case-control study of MI and smoking, using
hospitalized MI patients as cases will have alpha lower than beta (exposed cases are less available to
study than are nonexposed cases). This bias will produce a distortion in the odds ratio that will
understate a true association between smoking and MI (i.e., negative bias).
The assignment for this lecture has an exercise that asks you to apply this conceptual framework to a
detection bias issue involving endometrial cancer and estrogen. The basic issue is that use of
estrogen might lead to uterine bleeding, which would result in a woman seeking medical attention
and receiving a dilation and curettage (D&C). If an occult (asymptomatic) endometrial cancer were
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www.epidemiolog.net © Victor J. Schoenbach 2001 10. Sources of error - 297
with Joellen Schildkraut and Wayne Rosamond, rev. 5/11/2001, 5/16/2001