Gulnar Azevedo Silva Mendon9a
Selection of subjects in case-controls studies
Instituto Nacional do Ciincer e Instituto de Medicina Social-
UFRJ
T he selection of cases and controls seem s to be the m ost
difficult question in planning and conducting a case-control
study and rem ains essential regarding the practical
require-m ents for all the steps of this kind of epiderequire-m iological design.
In the lim it, it w ill be the indicator that can m easure validity.
T he presence of selection bias m ay influence the results of a
given study, m aking the observed association betw een the
factor under investigation and disease stronger or w eaker than
t6he actual association.
S election bias m ust be considered for both the case and
control series, since its related to their com parability. F or
B reslow and D ay selection bias can arise w hen cases and
con-trols are included in or excluded from a study because of som e
characteristics they exhibit are related to the exposure factor
being studied. T his kind of bias according to the type of
case-control study, the disease, the risk factors and other factors.
F requently they occur w hen the exposition status is
influenc-ing the selection of individuals differently for cases and
con-trols. T he probability of being selected m ight be the sam e for
both groups of com parison in the source population in order
to prevent selection bias.
S om e specific tim es of selection bias should be outlined
here as the B earkson's bias, w hich is caused by an increased
tendency for persons w ith m ultiple diagnosis to seek or enter
treatm ent and then fall into study populations draw n from
treat-m ent sources. In the specific situation there different
prob-abilities for those w ith the index disease under study and for
individuals w ith the exposure. T he selective survival bias is
another type of bias that can occur diferentially for cases and
controls and is associated w ith the disproportionate loss of
eligible subjects before selection.
D efinitionof cases
In the case-control design, tH e m ain aim is to ensure
that all true cases of a given population have the sam e
prob-ability of taking part of the study. T his pressupose a standard
definition of the disease being studied and all logistic
pro-ceedings to find the cases. T he source of cases, in general,
are based on hospital or clinic lists, death certificates and
spe-cial health records as cancer registries or disese surveillance
system s. T he criteria of elegibility should be applied equally
to potential cases and controls. T he definition of a "case"
in-volves the establishm ent of objective criteria for diagnoses
and a statem ent of elegibility criteria of selection fo
individu-als. T he exclusion criteria depends on the definition of
expo-sure and m ight be very w ell stablished at the exclusion
crite-ria depends on the definition of exposure and m ight be very
w ell stablished at the beginning of the study. T here are som e
circunstances that can lead to a detection bias. It occurs w hen
the disease in study is nott detected (even at death) or, in the
other hand, w hen individuals of high risk have m ore
inten-sive m edical surveillance.
C ontrolS election
It seem s to be a consensus am ong epidem iologists that
the choice of controls is the m ost difficult judgem ent in
case-control research. R othm an believes that the central aim of
control recruim ent raises on the assum ption that participate
as controls in a given study, should be potential cases in the
sam e study. T his condition, how ever, is som etim es hard to
m eet in a research protocol. T he principle that the cotrols
should be selected from the sam e risk population of the
se-lected cases is intuitively acceptable. W acholder et al. have
p ro p o s e d th re e a s p e c ts th a t m u s t b e c o n s id e re d in th e s e le c
-tio n o f c o n tro ls : th e s tu d y b a s e , th e c o n fo u n d in g e lim in a tio n
a n d th e c o m p a ra b le a c c u ra c y . If th e re s e a rc h e r a c h ie v e s th e s e
th re e a s p e c ts th e n th e p o s s ib ility o f s e le c tio n b ia s , c o fo u n d
-in g a n d -in fo rm a tio n b ia s w o u ld b e re d u c e d .
In o rd e r to a v o id fu tu re p ro b le m s w ith c o n tro ls s e le c
-tio n th e re s e a rc h e s m ig h t d e fin e w h ic h is th e b a s e p o p u la tio n
a n d ta rg e t p o p u la tio n fo r th e s tu d y w h e n th e re s e a rc h is b e in g
p la n n e d .
T y p e o f C o n tro ls
U s u a lly , c o n tro ls a re s e le c te d a m o n g h o s p ita l o r c lin ic
p a tie n ts o r a m o n g th e g e n e ra l p o p u la tio n . T h e lo g is tic d
iffi-c u ltie s to a iffi-c iffi-c e s s ra n d o m ly p o p u la tio n c o n tro ls p o in ts to th e
n e e d o f u s in g o th e r ty p e s o f c o m m u n ity c o n tro ls a s frie n d s ,
n e ig h b o u rs o r re la tiv e s .
H o s p ita l o r c lin ic c o n tro ls
D u rin g a lo n g tim e , a n d e v e n n o w a d a y s , th is ty p e o f
c o n tro l h a s b e e n la rg e ly u s e d . S o m e o f th e a d v a n ta g e s o f th e
h o s p ita l c o n tro ls la y o n th e fa c ts th a t th e y w o u ld c o m e , h y p o
-th e tic a lly , fro m th e s a m e p o p u la tio n b a s e th a t g e n e ra te th e
c a s e s a n d th a t th e q u a lity o f in fo rm a tio n w o u ld b e e q u iv a le n t
to th a t o f c a s e s . It is n o t o b v io u s ly , h o w e v e r, th a t th e c a s e s
a n d c o n tro ls fro m th e s a m e h o s p ita l o r s im ila r w o u ld h a v e th e
s a m e p o p u la tio n b a s e . T h e b e s t w a y to c h e c k if th e h o s p ita l
c o n tro ls a re re p re s e n ta tiv e o f th e c a tc h m e n t p o p u la tio n is
v a lid a tin g th e ir c h o ic e b y c o m p a rin g th e m w ith c o m m u n ity
c o n tro ls . U n fo rtu n a tly , th is is n o t p o s s ib le in e v e ry c a s e c o n
-tro l s tu d y .
P o p u la tio n c o n tro ls
T h is w o u ld b e th e id e a l c h o ic e o f c o n tro l if s o m e o f th e
p rin c ip le s in s e le c tin g c o n tro ls c a n b e a s s u re d a s : th e g u a ra n
-te e o f th e s a m e s tu d y b a s e in th e tw o g ro u p s o f c o m p a ris o n ;
th e s ta b lis h m e n t o f p re c is e c rite ria o f e x c lu s io n a n d th e p o s
-s ib ility o f e x tra p o la tio n to th e b a s e th e d is trib u tio n o f e x p o
-s u re -s in th e c o n tro l-s fo r p u rp o -s e -s -s u c h a -s c a lc u la tio n -s o f a b
-s o lu te o r a ttrib u ta b le ris k . T h e re a re s o m e d is a d v a n ta g e s o f
u s in g p o p u la tio n c o n tro ls s p e c ia lly , if th e c a s e a s c e rta in m e n t
is in c o m p le te a n d w h e n th e s a m p le o f th e s tu d y b a s e is im
-p o s s ib le to b e o b ta in e d .
N e ig h b o rh o o d s c o n tro ls
In th a t s p e c ia l ty p e , s o m e o p e ra tio n a l p o in ts a re e a s ily
s u c c e e d a s a c o o p e ra tiv e p a rtic ip a tio n o f th e c o n tro ls a n d th e
a b ility to m a tc h o n v a ria b le s re la te d to s o c ia l c la s s . T h e y a re
u s u a lly c h o s e n n o n ra n d o m ly w ith in a g e o g ra p h ic a l s e t w h a t
m a y le a d to a n in d e p e n d e n c e o f th e s e le c tio n p ro c e s s a n d th e
e x p o s u re . T h e u s e o f n e ig h b o rh o o d c o n tro ls m a y le a d to " o v e
r-m a tc h in g " o n th e s tu d y e x p o s u re if th e e x p o s u re is re la te d to
re s id e n c e .
F rie n d c o n tro ls
T h is s e e m s to b e a n a ttrra c tiv e a lte rn a tiv e fo r c a s e c o n
-tro l s tu d y in te rm s o f re s o u rc e s a n d lo g is tic s a s p e c ts . T h e y
c a n b e s e le c te d fro m a lis t o f frie n d s o r a s s o c ia te s o b ta in e d
fro m th e c a s e . A s th e c a s e a n d frie n d c o n tro l w ill h a v e s im ila r
s o c io e c o n o m ic s ta tu s , th e b ia s e s d u e to s o c ia l c la s s d a re
re d u c e d .T h e y c a n a ls o le a d to o v e rm a tc h in g re g a rd in g fa c
-to rs th a t d e p e n d s o n life -s ty le a n d o c c u p a tio n a l e x p o s u re s .
F rie n d c o n tro ls m a y b e u s e fu l in s o m e c irc u n s ta n c e s , s u c h a s
th e s tu d y o f g e n e tic a l d is o rd e rs .
O th e r ty p e s o f c o n tro l a re a ls o e m p lo y e d in e p id e m io
-lo g ic a l s tu d ie s b a s e d o n d e a th c e rtific a te o r d is e a s e re g is try
lis ts . T h e d e a d c o n tro ls m ig h t b e u s e d w h e n th e c a s e s in th e
s tu d y b u t d ie b e fo re b e in g in te rv ie w e d (6 ). T h e in fo rm a tio n
m ig h t b e o b ta in e d fro m a " p ro x y " in fo rm a n t.
