BOOKS OF RtfiDIfGS - PAHO/WHO

VoL XIX. N. a
- 102 -
f tdie outeonie of interest delries two
distinct categories (lfor exunple, death versus
surival). outcome rates in different
groups can, in theory, be compared by the
case-ontrol ("retrospective") method." ' & '
This method inivolves the election of a
series of earso (btho~ persons experiencing
the outo«me event) and the selection of a
reference series ofnoncases ("onrols" or
"referents") for comiparison to the case
series. Vasious itaitica4l techniques allow
estimion of differences in outiom occurrence
between groups based on the
cae-onatwl data'-provided the casecontrol
study satisfies certain conditions
requised for the validity of results. A primary
advantage of the case-control method
is that the number of subjects necessary is
usually much les tan that required for a
prospective comparison.' In the monitorlng
exnple, a cas~o ntrol study would
require fewer than 400 deliveries (200
cases and 200 controls) to detect a 50 per
cent reduction in the neonatal death rute
due to monitoring with 90 per cent probability
(power), at a 0.05 significance level t
(assuming a 50 per cent monitoring rto in
the target population).
More than 20 yean ago. Cornfield and
Haenszel' summrized the two critical
represmntativeness' conditions that were
thought necessary for the validity of a
cse-control study: 1) it should be "possible
to enumerate all new cases of a disease.
or a representative sample of them, without
haviig to doserve all the isdlviduals ¡i
the population"; and 2) "the sample of individuals
not developing the dierse ithe
contrulsl Ishould supplyl an unbiased estimrte
of the prevalence of the chtraceristic
under study among the entire nondiseased
population of interest. The authors
then went un tu lamuint that most retrospective
studies of dtit day had dificulty
assuring the saislacwtion of the seond reqOtler authors of
that period put forth simnilar criteria.u'
More recent writers have cxpressed related
criteria lor the validity oícal-cmotrol
CASEV-COXNlTOL MTHOD
resultl in a samplinig theory fraunework,?
while other authors have rejected representativeness
criteria and ilnstead have
empha>sized the importance ofcompanbility
ofcases and controls." A conimon feature
of all these points of view. however. is
un emphasis on the importance of "section
validity" in casfcontrol studies, in
that subject selection should be unbiased
in order to achieve valid results.
aIn the time since the early commentaries,
genend resevatioa s onceming the
utility of the caa.control method have
been put forth, largely o the grouds that
representativeness or selection validity
conditions ar difficult to fulfll or auure in
ordinary practice." Thbe purpose of our
commentary is to offer an appreciation of
the particular suitability of the case-control
approach for many problems in medical
cae evaluation. We will discuss why the
clasic problems of selection validity are
frequently absent in medical care evaluation
settinr. and how odither common problemns
ofordinary case-control resarch will
often be miimizod as well.
Coalidera~ Favorina the
CaM-Cotil Method in .
Evaluadon Sem~is
bepresenaatveanrs ad Se~oo
Co.sldodnlooe
A menlioned above, problem of selection
bias and subject nonrepresentativeness
haive loed o a curin degree of dinstrus
ofl s-control methods among many clinical
researchers. Yet it is precisely in the
realm of preliminary comparisons or
evaluations of medical care or medicld
teehnology tht the -control metdihod is
most often immune to such problems.
This is because many evaluation setting
involve amall, well-defined turget populabions,
such as a hospital inpatient population
or several co¡nbsied hospital pupulations.
In such settings it is usually possible
to enuumeraite the entire target population.
selecL representative wmlples froinm hud
a