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Biocultural Influences in the Analysis of Secular Trends
Claire C. Gordon and Thomas M. Greiner
In a human-engineering setting, anthropometric data col lection and
statistical analyses focus on target populations—the aggregates of persons
ho ill utili!e a particular materiel system or design. Because designs
may "e in use for se#eral decades after their introduction, secular trends in
the anthropometric distri"utions of target populations are of great concern
to the human-factors engineer. These target populations are formed
primarily through cultural processes that sample indi#iduals from $parent$
"iological populations. %nli&e many "iological populations, temporal
analyses of target populations lac& an a priori "asis for assuming
"iocultural continuity. To de#elop applied secular-trend models, it is thus
necessary to or& around this limitation. 'ne approach to this process ill
"e illustrated "y constructing secular-trend models for the target population
of the %.S. Army.
Secular-Trends Models
Secular trends are fre(uently #ieed as "iological responses to a
changing en#ironment. Impro#ed socioeconomic status, for e)ample, is
generally associated ith "etter health care and nutrition, and greater at-tainment of genetic potential for groth. *erein lies the more traditional
realm of secular-trends analysis. 'nly "iological populations undergo
secular change as a direct response to a changing en#ironment. *oe#er,
secular changes in "iological parent populations can influence the anthro-
pometric distri"utions of engineering target populations. Thus, secular
trends o"ser#ed in the army ill "e due, in part, to secular trends in the
%.S. population.
Target populations are "uilt through the actions of culturally determinedsampling processes, or $cultural filters,$ hich determine population
mem"ership. Thus, secular trends of target populations may "e modeled as
the interaction of cultural and "iological processes +see igure that
change parent populations and influence their relati#e contri"ution to the
target population. Army cultural filters inc lude the effects of recruiting
strategies, entrance re(uirements, and #arious societal opinions regarding
the merits of military ser#ice. Cultural filters, hoe#er, can also change o#er
time. This change might "e e)pressed as a change in recruiting strategies
//
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Time
Entren
ce
Standa
rds
Cultural
Filter sCultur
al
Attitud
es
Cultural
Filter sGende
r
Selecti
on
t
t
U U
igur e . A
model of %.S.
Armysecular
trendinfluences.
as
related to
racia
0A1A B%223TI0 4 /4
Time 1 Time 2
U.S. Army 1
t
U . S . A r m yRetention Standards
t
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l5eth
nic
grou
ps, a
chan
ge in
entra
nce
re(ui
reme
nts,or a
chan
ge in
the
cultu
ral
$ima
ge$of
the
soldi
er
+leg
al et
al.
677a,
677
"8
Grei
ner
and
Gord
on
66/
. In
this
ay,
secul
ar
trends in
a
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targe
t
popu
latio
n
can
resul
t
from
chan
gesin
ho
the
targe
t
popu
latio
n iscreat
ed,
apart
from
any
secul
ar
trends
that
may
"e
occu
rring
in
the
pare
nt
popu
latio
ns.
T
raditional
secu
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lar-
trend
mod
els
assu
me
"ioc
ultur
al
conti
nuityo#er
time.
In
the
appli
ed
setti
ng,this
assu
mpti
on
must
also
"e
9ustified
for
the
pro9e
cted
futur
e.
The
oper
ation
of a
cultu
ral
filter
creates a
com
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plica
tion
in
that
targe
t
popu
latio
ns
usua
llydo
not
fulfill
this
re(ui
reme
nt.
Thiscom
plic
ation
can
"e
mitig
ated
through
ident
ificati
on
and
sepa
rate
mod
eling
of
tar-
get
popu
latio
nsu"s
ets
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that
more
clos
ely
meet
the
assu
mpti
on of
"ioc
ultu-ral
conti
nuity
. The
ident
ificati
on of
these
su"s
ets,
ho
e#er,
re(ui
res a
detailed
loo&
at
the
cultu
ral
filter
s
oper
ating
in
the
form
ation
of the
targe
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t
popu
latio
n
+see
Ta"l
e .
C
hang
es in
theanthr
opo
metri
c
re(ui
reme
nts
for admi
ttanc
e
finto
the
army
can
direc
tly
influ
ence
anthr
opo
metri
cchan
ge
o#er
time.
*o
e#er,
an
e)amina
tion
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of
army
regul
ation
s
+A:;
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, this
sour
ce of
chan
ge
can
"e
dism
isse
d as
"eing
inco
nse(
uenti
al.
A
#ari
etyof
cultu
ral
filter
s
migh
t actin
conc
ert to
alter
the
dem
ogra
phiccom
posit
ion
of
the
army
and
thus
indir
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ectly
influ
ence
anthr
opo
metri
c
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/; :AC3, 3T*0ICIT@, A0 *%MA0 30GI033:I0G
Table 1Filters Influencing Migration of Ciilians into t!e Army
. Anthropometric :e(uirements
/. ifferential :ecruitinga.Gender ".:acial5ethnic groupc.Age
4. Secondary actorsa.3conomic status and incenti#es".3ducational re(uirements for military 9o"sc.3ntrance e)am scores
change o#er time. A com"ination of o#ert military recruiting strategies and
co#ert cultural opinions can effect change through differential selection "y
se), racial5ethnic group, and age. The effects of this type of selection can
"e addressed in secular-trend studies "y di#iding the sur#ey data into ap-
propriate demographic su"groups and modeling each separately. Changes
in selection criteria for se), race5ethnicity, and age can then "e in-
corporated in secular-trend models "y #ieing the target population as acomposite of these parts and utili!ing eighted estimates.
A third type of cultural filter might "e selection on a cultural factor op-
erating ithin demographic su"groups that correlates ith, "ut is not di-
rectly related to, anthropometric dimensions. 'ne e)ample of this might
"e preferential selection "y le#el of education. 3ducation le#els are often
related to socioeconomic status, hich in turn is associated ith a host of
"iological #aria"les that influence anthropometric distri"utions +legal et
al. 677a, 667"8 Garn et al. 68 *arlan et al. 677. The strength of these types of associations, hoe#er, #ares across cultural groups
+legal et al. 677a, 677"8 Meredith 6>. In any case, educational data
are not routinely a#aila"le for army anthropometric sur#ey su"9ects. Thus,
only general trends of educational selection can "e discussed. 'ther
cultural criteria might also "e at or& that ha#e no measura"le surrogate.
The action of the cultural filter in this fashion can therefore not "e
controlled or anticipated, and this constitutes a primary limitation in all
applied secular-trend models.
Analytical Methods
3ach sur#ey represents a snapshot of anthropometric #ariation ithin
the army population +see Ta"le /. To lin& these data sets, indi#iduals are
grouped into cohorts that represent fi#e-year "irth inter#als. olloing pro-
cedures outlined "y Greiner and Gordon +66/, the mean #alue for each
"ody dimension is calculated for each cohort. These #alues are then ad-
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Table 2
Birth Year Cohorts and Subjects' Ages by Survey Year
Cohort
Subjects' Ages When Measured
Birth Years1946 1966 1977 19
1 1!"9 !7# 19$"94 !#"!6% 19!"99 47"!14 19$$"$4 4#"46 6#! 19$!"$9 %7"41 !7"616 191$"14 %#"%6 !#"!67 191!"19 #7"%1 47"!1 ! 19#$"#4 ##"#6 4#"46 !%"!79 19#!"#9 #1 %7"41 4"!# !91$ 19%$"%4 %#"%6 4%"47 !4"!11 19%!"%9 #7"%1 %"4# 49"!%1# 194$"44 ##"#6 %%"%7 44"41% 194!"49 17"#1 #"%# %9"4%14 19!$"!4 16 #%"#7 %4"%1! 19!!"!9 1"## #9"%%16 196$"64 17 #4"#
17 196!"69 19"#%1 197$"74 1
justed &or age"reated change and sub(itted to regression anaysis) *o ac"
co((odate very arge di&&erences in sa(+e si,es- each cohort's (ean
vaue can be .eighted by cohort si,e in the statistica anayses)
*hese variate regressions +roduce the &oo.ing generic e/uation0
AA 2 a C+ b
3n this inear e/uation- b is the y"interce+t constant and a is the so+e o&
the ine) Because this e/uation describes the reationshi+ bet.een cohort
(C) and an age"adjusted anthro+o(etric di(ension AA5- the ter( a isthe rate o& secuar change +er &ive"year +eriod)
Whie a ongitudina +ers+ective &or secuar"trend anayses can be ob"
tained by ining the data &ro( t.o or (ore anthro+o(etric surveys- their
(easuring techni/ues (ust be care&uy scrutini,ed) ny di(ensions
(easured through stricty co(+arabe techni/ues can +rovide data &or the
anaysis) 3n addition- the de&inition o& +o+uation subgrou+s (ust be co("
+arabe) 8urther(ore- a 3 reevant birth years (ust be broady re+resented
in the avaiabe data) When these restrictions are a++ied to )S) Ar(y data
bases- their co(bination +er(its a (ae anaysis co(+rised o& &our ra"
cia:ethnic grou+s .hites- bacs- ;is+anics- and Asian:?*3= 1% #!
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#6 @AC?- ?*;=3C3*Y- A= ;MA= ?=3=??@3=
igure /. *ead "readth +mm "y "irth-year cohort.
and a female analysis of only to racial5ethnic groups +hites and "lac&s.Dhen insufficient data limit modeling to only some segments of the target
population, this clearly induces some limitations to the application of re-
sults. *oe#er, in this case "ecause hites and "lac&s together comprise
64.; percent of the females in the army +Gordon et al. 676, this limitation
may not "e too serious.
Temporal Sta"ility of Secular-Trends Models
The cru) of secular-trend analysis lies ith the construction of relia"le
and temporally sta"le models for each of the identified target population
su"groups. An important aspect of this is the re#ie and specification of
any temporally unsta"le cultural influences. A parallel e)amination of esti-
mated secular trends in %.S. Army males and females pro#ides an
e)ample of the differential operation of cultural filters and their relati#e
influence on model relia"ility.
Three ma9or sur#eys of army omen are a#aila"le to "uild seculartrend
models. *oe#er, hen these data are plotted against "irth-year cohort,
there is a consistent separation of the 6;> data set, most dramatically seen
in igure /E a scatterplot of head "readth #ersus "irth-year cohort. The
special nature of se#eral factors operating on the 6;> army omen may
e)plain this separation. irst, the "ul& of these omen e)perienced their
groth years during the Great epression of the 64
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Table Recent Milestones for U.S. Army "omen
6> 1u"lic 2a 6 data set represent the forma-
tion of the Domen?s Army Corps in response to the national emergency of
Dorld Dar II +DDII. As an e#ent, DDII represents a comple), uni(ue, and
discontinuous cultural filter hose direct and indirect influences on anthro-
pometric #alues are difficult to discern. There are insufficient data to $cor-rect$ for the uni(ue influence that DDII had on army omen?s anthropomet-
ric distri"utions. Therefore, the only alternati#e is to ac&noledge that the
6;> sur#ey data represent a separate target population that cannot, at this
time, "e lin&ed to the later sur#eys in a larger trend model. 'ngoing re-
search in this area ill attempt to identify social and demographic differ-
ences "eteen the DDII-era Domen?s Army Corps, their ci#ilian counter-
parts, and their military $descendants.$
Dhi le analysis of the remaining to data sets +6 and 677 resultsin more sta"le secular-trend models, their lac& of time depth may limit
model relia"ility. The 6 and 677 sur#eys o#erlap for all "ut to "irth-
year cohorts. Therefore, these data do not pro#ide long-term information
on ho cultural processes might "e affecting the population. 0e#ertheless,
changes in the cultural filters affecting army omen might "e associated
ith e#ents "eteen 6> and 67 +see Ta"le 4.
In effect, "eteen 6> and 67, there as a "roadening of the cul-
tural selection criteria that influenced omen?s participation in the army.These changes may ha#e indirectly influenced anthropometric dimensions
e#en though no specific change in anthropometric selection criteria oc-
curred. It is difficult to del ineate indirect cause and effect relationships "e-
teen cultural actions and anthropometric trend. *oe#er, if it could "e
shon that ne cultural filters "ring a"out a shift in the socioeconomic ori-
gins of omen recruits, then inclusion of socioeconomic le#el, or some
pro)y for it, may "e needed to enhance model relia"ility. 'ne possi"le
measure that might "e used is the association of socioeconomic le#el ithle#els of education. uring the period of 6> to 67, for e)ample, accept-
0A1A B%223TI0 4 /
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28 RACE, ETHNICITY, AND HUMAN ENGINEERING
a"le le#els of education ere "roadened and entrance le#els ere loered
for omen recruits. *oe#er, these changes did not palpa"ly affect the di
stri"ution of omen?s education le#els +Soldier Support Center 67/8 %.S.
Army Military 1ersonnel Center 6. Therefore, education +e#en if it erea useful pro)y for socioeconomic le#el may not "e useful in e)plaining the
secular trends of army omen.
Because the original 6;> sur#ey data for men are not a#aila"le, the
6>> and 677 sur#eys of army men ha#e "een used to "uild secular-trend
models +Greiner and Gordon 66
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ithin the army, despite an o#ert shift in recruiting goals in 66 to targethigh school graduates +*unter and 0elson 67/8 Soldier Support Center 67/8 Dhite and *ose& 67/. Thus, shift in educational le#el ould not
seem to "e an important influence in the secular trends of army males.More specific cultural influences may "e acting in the case of Asian51acific Islander males. rom 6>> to 677 there as a significant in-crease in the num"er of foreign-"orn mem"ers of this group, causing a shiftfrom predominately American-"orn soldiers to predominantly foreign-"ornsolders, most of hom carne from the 1hilippines +Greiner and Gordon66/. The impact of this shift on stature trends in the %.S. Army Asian51a-cific su"group is readily seen in igure 4 and illustrates a classic case inhich the a#aila"le racial5ethnic classification is at too "road a le#el to pro-#ide sta"le estimates of future stature trends in this su"group. To impro#ethese estimates, population-specific data and models may "e re(uired for such heterogenous groups as Asian51acific Islanders, *ispanics, and 0ati#e
Americans. As indicated "elo, application of such models re(uires specificestimates of future minority group fre(uencies, and as :o"ert Dal&er haspointed out, their contri"ution to the army population is so small that theymay ha#e little impact on armyide parameter estimates regardless of theaccuracy of their indi#idual secular-trend models.
Model Applications
Models resulting from cultural filter research and su"group identifica-tions operate "y separately predicting #alues "y "irth-year cohort for eachdemographic su"group. These models assume "iocultural continuity ithineach of the demographic su"groups. 3stimates for future target populationsare "uilt "y com"ining indi#idual estimates for each su"group, eighted "y
appropriate demographic proportions. In this ay, population predictionschange according to their demographic ma&eup. An e)ample of this can "eseen "y predicting the mean #alues for stature of army males in the year /
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4< :AC3, 3T*0ICIT@, A0 *%MA0 30GI033:I0G
Table *Secular Trends in U.S. Army Male Stature
Trend
:acial53thnic Group +mm5cohortDhites 4.Blac&s .;>*ispanics =.4; Asian 1acific islanders -=.>;
@ear 1opulationa
Mean
Stature
677 Current 1opulation => mm
/
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Table )Accommodation :ro>ections for an Army 4elmeta
0o emographic Change More @ounger Cohorts
Si!e 677 /.
Dith no demographic change, e ould e)pect to see the mean #alue of
head "readth to decrease "y .> mm, head length to increase "y 4 mm and
head circumference "y ; mm. To of these changes are still "elo the
alloa"le o"ser#er error. @et, these small changes are so influential that the
current design could concei#a"ly fail to accommodate 4/ percent of the
target population. Thus, e#en unspectacular rates of change can interact
ith close-fitting designs and greatly influence the accommodation of future
target populations.
Conclusions
Applied secular-trend models are rarely "uilt from data gathered ith
the aim of model construction. *oe#er, the application of secular-trendmodels does not re(uire that all sources of change "e identified and under-
0A1A B%223TI0 4 4
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4/ :AC3, 3T*0ICIT@, A0 *%MA0 30GI033:I0G
stood. Instead, their application relies upon the assumption that past trends
ill continue finto the future. Because this assumption ill e#entually pro#e
false for any population, it is important to understand a model?s limitations
"efore it is applied. An understanding of the mechanisms that con-tri"ute tosecular trends merely pro#ides some a"ility to anticipate the sources of
insta"ility in the model. 'nce secular trends are modeled, hoe#er, their
rele#ance to design can only "e determined "y the sensiti#ity of the design
criteria.
:eferences Cited
Boc&, :. ., and :. C. Sy&es
676 3#idence for Continuing Secular Increase in *eight Dithin amilies in the %nitedStates. American ournal of *uman Biology E;4-;7.legal, J. M., D. :. *arlan, and . :. 2andis
677a Secular Trends in Body Mass Inde) and S&infold Thic&ness ith Socioeconomicactors in @oung Adult Men. American ournal of Clinical 0utrition ;7E=;;-==.
677" Secular Trends in Body Mass Inde) and S&infold Thic&ness ith Socioeconomicactors in @oung Adult Domen. American ournal of Clinical 0utrition ;7E=4=-=;4.
Garn, S. M., S. M. Bailey, 1. 3. Cole, and I. T. T. *iggins6 2e#el of 3ducation, 2e#el of Income, and 2e#el of atness in Adults. American ournal
of Clinical 0utrition 4
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Dhite, . 1., and . :. *ose&67/ The Analysis of Military Manpoer Issues. In Military Ser#ice in the %nited States.
3. Scocroft, ed. 1p. ;>. Technical :eport /-=-C3. 0atic&, MAE %.S. Army Huartermaster :esearch and 3ngineering Center.
0A1A B%223TI0 4 44