Capítulo 2 Elementos del sistema político electoral en México
2.3 Principios rectores de la función electoral
n T| X 1.25(design effect)
O U T C O M E S 4 m acro -areas 4 m acro -areas
C aries-free (D M F = 0) 1,760 2,200
D e n ta l T raum a 1,064 1,330
D e n ta l pain 328 410
T h e safest ch oice fo r sam ple size, w h en a range o f o u tc o m es o r p rev alen ce is to
be stud ied, is th e largest one. T his is a ra th e r co n serv ativ e choice, b ecau se th e
p o w er o f th e te st will be larger th a n 90% fo r so m e o u tc o m es. I t m eans it w ould
be m o re th a n 90% likely to d e te c t certain e sta b h sh e d rates. C onversely, it
w ould resu lt in a loss o f p recisio n o r co n fid e n ce o r b o th fo r som e o u tc o m es,
w ith a sam ple size sm aller th a n th e value calculated. T h u s, th e to ta l sam ple size
ch o sen in this study was equal to 2,200 children.
3.4.5 - D ata co llectio n du ring th e survey stud y
CUnical prim ary data req u ired to ch aracterise individual oral h ealth o u tc o m es,
as well as individual covariate data w ere co llected b ased o n th e B A S C D ’s
stan d ard ised p ro c e d u re s (see A p p en d ix I II , p. 302, fo r d en tal caries an d th e Hst
o f in stru m e n ts and m aterials used in th e survey study; A p p en d ix IV , p. 307, fo r
d en tal traum a; A p p en d ix V, p. 308, fo r d en tal pain; A p p en d ix V I, p. 310, for
th e O ral H ealth A ssessm en t F o rm , w ith individual covariates; and A p p en d ix
V II, p. 313, fo r S cheduled p ro g ram m e o f diary activities).
3.4.5.1
-Covariates
A cov ariate can b e a variable p red ictiv e o f th e o u tc o m e u n d e r study, o r it may
be a c o n fo u n d in g variable o r effect m o d ifier (Last, 1995). O p tim al design
strategies fo r p o ten tially c o n fo u n d in g covariates are n o t th e sam e as for
exp o su res, in w hich one seeks to m axim ise b e tw e e n -g ro u p s d ifferences fo r th e
ex p o su re ex p lan ato ry variables. F o r covariates, b e tw e e n -g ro u p h etero g en eity is
M ethods 149
“ co m p arab le” regions w ith sim ilar cov ariate d istrib u tio n s. T h is p rin cip le is
analogous to th e p rin cip le o f seeking co m p arab le ex p o sed an d u n ex p o sed
c o h o rts fo r individual-level studies (G reen lan d , 1992).
T h e aims o f th e covariates data co llectio n an d analysis w ere b o th to co n firm
b etw ee n areas’ sim ilarities o f individual ch aracteristics o f th e sam ple, an d to
te st c o m p e tin g h y p o th esis fo r th e ex p lan ato ry variables. T o recap itu late, w hen
th e m a tte r o f co m m o n d em o g rap h ic ch aracteristics was taken in to acco u n t,
m icro -areas an d individuals w ere p re -m a tc h e d by th e follow ing aspects:
(i) Urban location and socio-economic situation o f the communities’, th ey w ere th e
fa rth e st p e rip h e ra l areas o f th e city. M icro-areas w ere c h o sen to refle ct a
co n tin u u m o f social d ep riv a tio n , w ith c o n tra stin g levels o f h ealth y
policies im p lem en tatio n . In th e p re s e n t study th e p o ssib ility o f a so c io
eco n o m ic “ cro sso v e r” effect b etw ee n th e g ro u p s is unhkely an d can be
totally d isreg arded since th e so cio -ec o n o m ic variables w ere assessed o n an
individual- as well as g ro u p - level;
(ii) Location and type o f the schools surveyed: all ch ild ren stu d ied cam e from
p e rip h e ra l p u b h c schools. As already stated , th e p e rip h e ra l pu blic scho ols
co m m o n ly e n ro l ch ildren fro m below p o v erty th re sh o ld . P ublic sch oo ls
w ith no tu itio n fees are usually p re fe rre d by socially u nd erp riv ileg ed
g ro u p s, w hereas p riv ate sch o o ls w ith high tu itio n fees re c ru it th e ir
stu d e n ts fro m rich fam ihes. T h u s, co m p arab le so cio -ec o n o m ic status
am o ng th e study subjects w ould be in su red ;
(iii) A.ge o f the children: all children w ere tw elve-years-old. T h e age o f the
ch ild ren was d efin ed by th e ir ch ro n o lo g ical age o n th e day o f
exam ination.
T his ab ov e jud gem en ts have b een re fe rre d to as prima facie m a tch in g (K reu ter et
al., 1999). H ow ev er, th e a n te rio r p re c a u tio n s w ere n o t eno ug h. W hen
co m m u n ities are “ n aturally” o ccu rrin g , th eo retically , o th e r system atic
d ifferen ces could occur.
As a m eans to co n firm th e prima facie ju d g em en t o f th e co variates h o m o g en eity
M ethods 150
em p lo yed (D en zin , 1970; B ran n en , 1992; B radley, 1995), T h e ap p ro a c h used
th re e in d e p e n d e n t p ro ced u res:
(i)Literature review on healthy public policies indicators, leading to a c o n te n t
analysis o f th e official m aterial describ in g in te rv e n tio n s and evaluations
held in th e m icro -areas. T h is p ro c e d u re is p re s e n te d in th e p an el study
m e th o d s , at th e co m in g section ;
(ii) Interviews with key informants within the areas, to o b ta in co n sen su s scores on
in d icato rs o f th e areas, w hich is also b e st d escrib ed at th e p an el study;
(iii) Individual information on covariates, co llected by q u estio n n aires du rin g th e
survey study.
F o r th e covariates, it tu rn e d o u t to be com plex to try to in teg rate m any aspects
o f th e reaUty in m o re th a n o n e level o f in te rp re ta tio n . T h e re w ere caveats to
co n sid er. P o ssib le relatio n s b etw ee n th e m acro-level (com m unity) and m ic ro -
level (individual) claim ed fo r a very stric t v erificatio n o f c o n c u rre n t and
co m p etitiv e exp lanation s ov er social and biological d e te rm in a n ts o n oral health.
F o r exam ple, th e c o m m o n c o n fo u n d in g and bias d escrib e d in th e m edical
H terature are:
(i) A rtifac t bias o f official statistics g ath erin g p ro c e ss o r data analysis,
gen era tin g spu riou s results fo r th e assum ed ex p lan ato ry variables. T his
p ro b le m was dealt w ith an e n h an c ed m eth o d o lo g ical ap p ro ac h . So,
trian g u latio n was n o t used as sim ply eclecticism . I t was c o n sid ere d in
close rela tio n to th e q u estio n s and th e o rie s th a t allow to ch eck th e
findings fro m o n e level against th e findings fro m a n o th e r level o f
m e asu re m en t units. I t im p ro v es th e rig ou r an d vahdity o f th e data, and
the co n siste n cy o f th e results;
(ii) S election bias o f u n h ealth y individuals b ein g sim ply le ft b e h in d in th e
less privileged social gro ups. I t is unhkely in th e p re s e n t case th a t oral
h ealth o u tc o m es could be such an ex p lan atio n , in v e rtin g th e d irec tio n o f
d e te rm in a tio n b etw een so cio -ec o n o m ic statu s and d en tal disease;
(iii) Bias o f u n c o n tro lle d so cio -eco n o m ic d ifferen ces at th e h o u se h o ld level.
________________________________________________________________________M ethods 151
In ad d itio n , in d en tal-related featu res th e re m ig h t b e th e follow ing
c o n fo u n d e rs: a) w ater flu o rid atio n access; b) flu o rid ated to o th p a s te access; c)
place and freq uency o f d en tal care; d) dietary p a tte rn s.
T h e co variates w ere m easu red and c o n tro lle d fo r to th e e x te n t th a t they have
b e e n co n sid ere d im p o rta n t in th e specialised H terature an d w ere m easurable.
T h ey re fe rre d to: a) c h ild re n ’s family in co m e; b) em p lo y m en t o f c h ild re n ’s
fath ers and m o th e rs; c) ed u catio n o f c h ild re n ’s m o th e rs; d) c h ild re n ’s
b irth p la c e an d family geograp hical origin; e) c h ild re n ’s access to an d freq uen cy
o f utiH sation o f d en tal services; f) c h ild re n ’s access to an d utiH sation o f w ater
flu o rid atio n ; g) ch ild re n ’s access to and utiH sation o f flu o rid ated to o th p a s te ;
and , h) c h ild re n ’s Hving tim e in th e ir resid en tial area - th e individual ex p o su re
variable (see A p p en d ix V I, p. 310).
In an ecological (or area-based) study, o n e asp ect o f vaHdity, w hich is th e
p o p u la tio n m obiHty, can severely c o n fo u n d any analysis o f th e rela tio n o f area-
b ased e n v iro n m e n ta l c o n d itio n s and area-b ased h ealth o u tc o m es. I t is th e
q u estio n o f w h e th e r data are m easuring w h at it ap pears to m easure. It is
k n o w n , fo r exam ple, th a t residential m obiHty is h ig h est in th e m ig ran t areas o f
cities in d ev elo ping co u n tries (S tep hen s et al., 1994). C ro ss-sec tio n al data on
p rev alen ce do n o t tell a b o u t Hfetim e ex p o su re o f an individual to h is /h e r area
o f residence. In o th e r w o rd s, an individual m ay Hve m u ch o f h is /h e r Hfe in an
area w ith p o o r c o n d itio n s, b u t m ay m ove and b eco m e ill in a n o th e r area
alto g eth e r. I t is also vaHd fo r th e p ro b le m o f c o n ta m in a tio n o f data am on g
n e ig h b o u rin g areas.
Since th e 3*^*^ stage o f the sampHng in clud ed collecting cHnical data fo r each
sch o o lch ild , th e in s tru m e n t used also in clu d ed th e issue o f w h e th e r th e ch ildren
w ere Hving in th e area fo r lo n g en o u g h to be ex p o sed to th e e n v iro n m e n t
u n d e r c o n sid era tio n . B esides th e c h ild re n ’s sch o o l re c o rd s, o th e r c ro ss
check in g q u estio n s w ere used lo ok ing at th e c a tc h m e n t area o f th e ch ild ren and
M ethods 152
3.4.6 - O peration al procedures during th e survey stud y
T h e survey p ro c e d u re s in clu d ed p re -te stin g activities w ith th e ch o sen
in s tru m e n ts , o b ta in in g facihties and su pp hes w ith m un icipal a u th o rities, and
train in g th re e oral hygienists w ho assisted w ith clerical an d logistical su p p o rt.
A p ilo t study was d o n e du rin g tw o w eeks o f Jan u ary 1998, ju st b e fo re startin g
th e field research . D u rin g th e p ilo t study, th e O ral H ealth A ssessm en t
In s tru m e n t was te ste d and th e co n v en ien ce o f m o n ito rin g in tra-ex am in er an d
in ter-ex am in ers d iag no stic co n sisten cy was assessed. F ro m th e p ilo t study, it