Copyriqhl 1992 byThe Gerontologicol Society ofAmerica
l ournal of Geronto lo gy : P SYCH O LOGI C AL SCI ENC LS
1 9 9 2 .V o l . 4 7 , N o . 3 , P l 2 l 1 2 8
WhatDo Adult Age Differencesin the
Digit SymbolSubstitutionTestReflect?
Timothy A. Salthouse
School of Psychology, Georgia Institute ofTechnology
Resultsfrom three studiesare reported in which adults between18 and 84 years of age performed various versions of
the Digit Symhol Substitution Test. The frst study revealed that the age-related declines in digit-symbol pedormance
were largely independent of both the amount of education the participants had received and their self-reported health
status, and h'ere characterfued by a gradual shift in the entire distribution of scores with little age-related increase in
variance. The age relations were greatly attenuated after statistical control of a composite meosure of perceptual
comparison speed,however, implying considerable commonalities betweenperceptual comparison speedand what the
Digit Symbol Substitution Test measures. Two further studies indicated that young and old adults appeared to use
similar strategies to perform the task, and were nearly equivalent in the proportions of time devoted to writing the
responsesand searching the code table.
THE Digit Symbol SubstitutionTest consistsof a code
r tabledisplayingpairsof digitsandsymbols,androwsof
double boxes with a digit in the top box and nothing in the
bottom box. The task for the examineeis to use the code
with eachdigit, and
tableto determinethe symbol associated
to write as many symbols as possible in the empty boxes
below eachdigit. Although the test seemsquite simple, it
may be importantfor the understandingof adult age differencesin cognitionbecausescoreson it are highly correlated
age.In
of intelligence
andchronological
bothwith measures
fact, the relationswith intellectualmeasuresare so strong
that Wechsler has included it as one of the subtestsin his
adult intelligencescales.Correlationsof digit-symbol score
with full-scale intelligence score reported in the WAIS
(Wechsler,1955)and WAIS-R (Wechsler,l98l) manuals
range from .51 to .74, with relatively little systematic
variation in the magnitude of the correlationsacross the
rangefrom 18 to 14 yearsof age.
The relationsbetweenageand digit-symbolscoreare also
large,as analysesof the WAIS (Binen & Monison, 1961)
andWAIS-R (Kaufman,Reynolds,& Mclean, 1989)standardizationdatarevealedcorrelationsbetweenageand digitsymbol scoreof - .46 and - .54, respectively.Similar age
trendshavealsobeenreportedwith a symbol-digitversionof
the task in which digits are written insteadof symbols. To
illustrate, correlationsbetweenage and symbol-digit score
have been reportedto be - .6'7 for a sampleof 172 adults
(Emmerson,Dustman,Shearer,& Turner, 1989), - .67 for
a sampleof 125 adults(Gilmore,Royer, & Gruhn, 1983),
and - .17 for a sampleof 386 adults (Royer, Gilmore, &
G r u h n ,l 9 8 l ) .
In light of thesestrongrelationswith both intelligenceand
adult age, it seemslikely that the Digit Symbol Substitution
Test is measuringprocessesimportant to the relation between age and cognition. A potentially fruitful research
agendamight thereforeinvolve focusing on the causesand
of the processesresponsiblefor age-related
consequences
variationsin digirsymbol performance.
Three hypotheseshave been proposedin recent years to
accountfor age-relateddifferencesin digit-symbol performance. What might be termed the peripheralmotor speed
hypothesisattributesmost of the age differencesto an agerelatedreductionin the speedof making manualmovements,
such as writing symbols.This interpretationis bolsteredby
the fact that writing speedhas been found to decline with
increasedage (e.g., Binen & Botwinick, l95l), but it is
weakenedby its inability to explain the associationbetween
digit-symbolscoreand measuresof higher order cognition.
Severalsetsof empiricalfindingsreportedin the last l5 years
with the peripheralmotor speedhypothearealsoinconsistent
sis. For example, Erber (1986), Erber, Botwinick, and
Storandt(1981),Salthouse(1988), and Storandt(1976)all
measuredthe speed with which individuals could merely
copy symbols,in additionto the speedof substitutingsymbols for digits in the standardtest. The ratiosof the copying
measuresto the standardmeasureswere nearly identical for
young andold adultsin eachstudy [i.e., .44 for young adults
and .42 for older adultsin Erber (1986); .48 for young adults
and .47 for older adultsin Erber et al. (1981); .45 for young
adultsand .46 for olderadultsin Salthouse( I 988);and .50 for
young adults and .53 for older adults in Storandt(1976)1.
These results are therefore consistentin indicating that both
young and old adultsdevotedapproximatelythe sameproportion of their total performancetime to writing the symbols.
Another set of results suggestingthat writing speed is
probablya minor factorcontributingto the agedifferencesin
digit-symbolperformancederivesfrom studieswith a modified Yes/No version of the digit-symbol task. Both
Salthouse (1978) in a paper-and-pencilversion, and
Salthouse,Kausler, and Saults (1988) in a computeradministered version, found substantial age differences
when the researchparticipantwas simply required to indicate whether the displayed digirsymbol pair was correct
according to the code table. Age correlationsin the two
Salthouse
et al. (1988)studies,in which the decisionswere
communicatedby a keypresson a computerkeyboard,were
Pt2l
Pt22
SALTHOUSE
.54 and .56 - values quite similar in absolutemagnitude
(but opposite in sign becausethey representtime per responserather than number of responsesin a fixed time) to
thosereportedin the studiescited earlier.
A secondhypothesisproposedto accountfor the negative
relations betweenadult age and digit-symbol performance
invokesmemory limitationsas the critical factor. That is, it
has been proposedthat older adults may be lessefficient at
learningor rememberingthe associationbetweenthe digif
symbol pairsthan young adults,and consequentlythey must
engagein.more time-consumingsearchesof the code table
during the performanceof the task. This interpretationhas
the advantageof relying on a mechanism(memory limitations) that can be plausibly relatedto other cognitive functions and is consistentwith the well-documentedfindingsof
reduced efficiency of paired-associatelearning with increasedage [e.g., Salthouse
et al. (1988)recentlyreported
correlationsrangingfrom -. 15 to -.42 betweenage and
measuresof paired-associateperformancel. However, a
numberof resultssuggestthat this interpretation
is unlikely
to provide a completeexplanationof the negativerelation
between age and digit-symbol performance. Among the
relevant studiesis one by Erber et al. (198l) in which
substantialage differenceswere still evident even after all
the young andold researchparticipantshad learnedthe digitsymbol pairs to a criterion of perfect recall. There are also
severalreportsthat agedifferenceseitherremainconstant,or
evenincrease,acrossl0 (Erber, 1976),20(Grant,Storandt,
& B o t w i n i c k ,1 9 7 8 ) ,a n d e v e n 1 0 0( B e r e s& B a r o n , l 9 8 l )
repetitionsof the task. The additionalpracticeshould have
allowedmany of the digit-symbolpairsto be committedto
memory, and yet in noneof thesestudieswas therea significant reductionof the magnitudeof the age differenceswith
greaterpractice.Finally, Salthouse(1978)reportedthat the
relative age differencesremained constantwhen memory
loadwasminimizedby reducingthe numberof digit-symbol
pairs from 9 to 6 to 3 to l. If older adultsdevoteda larger
proportion of their performancetime to inspectionof the
code table than young adults, then reducing the number of
entriesin the code table should have benefitedolder adults
more than young adults. The fact that this did not happen
suggeststhat the two groups devoted approximately the
sameproportionof time to inspectionand searchof the code
table.
The third hypothesisregardingthe agerelationson digitsymbol performanceis that they are a reflectionof an agerelated slowing of many cognitive operations(Salthouse,
1985).That is, ratherthanrepresentinga specificage-related
deficit in one particularprocess,the agedifferencesin digir
symbolperformancemay be a consequence
of a ratherglobal
slowing of many perceptual, motor, and cognitive processes.Unlike the other hypotheses,this hypothesiscurrently has little directly relevantevidence,and insteadhas
receivedconsiderationin largepart becauseof the perceived
limitations of the other hypotheses.
Each of the precedinghypothesesis reexaminedin this
report. Different methodologiesare used to investigatethe
peripheral-motorand limited-memory interpretationsthan
those used in the past, and the relation of digit-symbol
performanceto performanceon othersimplespeededtasksis
examinedas a meansof investigatingthe hypothesisbased
on a relativelygeneralage-relatedslowing.
Study I
The rationalefor the first study was that if agedifferences
in digit-symbolscoresprimarily reflectan age-relatedslowing of many processingoperations,then a substantialproportion of the age-relatedvariance in digit-symbol performance should be eliminated by removing the variance
associatedwith other speededmeasures.The other measures
of speedexamined in this study involved comparisonsof
strings of letters or patternsof lines. Both tasks are very
similarto thoseassumedto measureperceptualspeed(e.g.,
Number Comparison, Identical Pictures) in that simple
(same/different)decisionswere to be made with respectto
whetherthe two membersof the pair were identical.
The availabilityof datafrom a relativelylarge (N : 9 l0)
sampleof adultsfrom a wide rangeof agesalso allowed the
exact natureof the age trendsto be investigated.For example, two possibilitiesare that with increasedage either (a)
the entire distribution shifts toward lower scoresor (b) an
increasingpercentageof people perform at relatively low
levels,whereasthe othersmaintainhigh levelsof functioning. Negativerelationsbetweenage and performancecould
be producedin each case, but the implicationswould be
quite different. A shift in the entire distributionwould
suggestthat most people are affected by the age-related
processes,whereasa bimodal patternwould imply that only
a portion ofthe relevantpopulationwas affectedby the agerelatedinfluences.
MErHoo
Subjects.- Researchparticipantswere recruited from
newspaperadvertisementsrequestingvolunteersto participatein researchprojectsconcernedwith memory and cognition. A total of 910 adultsbetween18 and 84 yearsof age
participatedin one of four separateprojects with between
220 and235 adultsin eachproject. (No analysesof the digitsymbol datawere describedin the reportsof theseprojects.)
The meanageof the 9 10 adults(547owomen)with complete
datarelevantto the currentpurposeswas 48.8 years(SD :
17.3).The meanyearsof educationwas 15.4 (SD : 2.5),
and the meanhealthrating on a 5-point self-assessment
scale
(l : excellent,5 : poor)was2.l (SD : 1.2).Correlations
of thesevariableswith chronologicalage were -.01 for
educationand .13 (p < .01) for self-raredhealth.
Procedure. - The WAIS-R Digit Symbol Substitution
Test was administeredalong with other testsof perceptual
comparison speed, memory span, working memory, and
various types of cognition. Measurescommon acrossthe
four projects, besidesthe Digit Symbol SubstiturionTest,
were: letter comparison,pattern comparison,computation
span, and listening span.The letter comparisonand pattern
comparisontestsconsistedof pagescontainingpairsof 3, 6,
or 9 letters,or patternscomposedof 3 , 6, or 9 line segments
.
One-half of the pairs were identical, and one-half were
different (created by changing one of the letters or line
DIGIT SYMBOL
segmentsfrom a correct pair). The task for the participant
was to classify each pair as SAME or DIFFERENT, by
writing an S or a D on a line betweenthe two membersof the
pair, as rapidly as possible.Trials with 3, 6, or 9 elements
were separatelytimed, and the scoresaveragedto provide a
single measurefor each type of comparison(letter or pattern). Becausethe time limits were 20 secondsper page in
the first projectand 30 secondsper pagein the otherprojects,
the scores from the participantsin the first project were
multiplied by 1.5 to makethem comparablewith thosefrom
participantsin the otherprojects.
The computationspan and listening span tasks were designedto assessworking memoryby requiringparticipantsto
rememberinformationwhile alsocanying out specifiedprocessing.In the computationspantask, arithmeticproblems
were presentedauditorily, and the task was to answer the
arithmeticproblem while also rememberingthe last digit in
each problem. Short sentenceswere auditorily presentedin
the listeningspantask, with participantsinstructedto answer
a questionaboutthe sentencewhile alsorememberingthe last
word in eachsentence.The numberof arithmeticproblemsor
sentencespresentedon a trial increasedfrom one to seven,
with threetrialsat eachsequence
length.An individual'sspan
was determinedby the highestnumberof digits or wordsthat
could be rememberedon at leasttwo of the threetrials for a
sequencelength,given that he or she was also conect in the
answersto the relevantarithmeticand sentencecomprehension questions.This latterrequirementensuredthat the scores
represented
both storageand processing.
REsulrs ANDDlscusstoN
Digit-symbolscoresin this samplecorrelated-.54 with
chronologicalage.This value is quite closeto thoseof the
studiescited earlier,and the regressionslopeof - .47 items
per year is also similar to the slope of - .43 reportedby
Emmersonet al. (1989) for the Symbol-Digittest. These
resultssuggestthat the presentsampleis probablyrepresentative of other samplesof adultswho participatein research
projectsof this type.
A multiple regressionanalysis with age and gender as
variablesrevealedthat both main effectsandtheir interaction
w e r e s i g n i f i c a n(ti . e . , a l l F ' s > 8 . 4 , 7 l < . 0 1 ) . S e p a r a t e
analysesof the age relationsfor men and women indicated
that the regressionequationswerel5.3l - .40 (years)with
an r'of .25 for men,and87.06- .54 (years)with an r' of .35
for women. The interaction is therefore attributableto a
higher initial level, but greater age-relateddecline, for
women than for men.
The distributionof scoresby decadeis illustratedin Figure
l It is apparentin thesedatathat the negativeage relations
are associatedwith a shift in the entire distribution toward
lower scoreswith increasedage. There is little indicationof
a tendencytoward bimodality with advancingage, and even
the varianceappearsto remain relatively constantfrom the
decadeof the 20s through the decadeof the 70s. Standard
deviations for the six distributionsillustrated in Figure I
w e r e 1 2 . 1 9 ,1 3 . 6 0 ,I 1 . 2 8 , 1 5 . 0 4 ,1 0 . 9 6 ,a n d 1 2 . 9 7 ,f r o m
the 20s throughthe 70s, respectively.
The two perceptualcomparisonspeedmeasureshad correlations with one anotherof .78, and the correlationbe-
P123
tweenthe two working memory measureswas .59. Composite scoresfor each construct were created by averaging the
z-scoresfor the letter comparisonand pattern comparison
measuresfor perceptualcomparisonspeed, and averaging
the z-scoresfor the computation span and listening span
measuresfor working memory. Correlationsof thesecompositemeasureswith agewere - .64 for perceptualcomparison speedand - .49 for working memory, and those with
digirsymbol score were .74 for perceptual comparison
speedand .56 for working memory.
A seriesof hierarchicalmultiple regressionanalyseswere
conductedto examine the magnitudeof the relations between age and digit-symbol performancebefore, and after,
statisticalcontrolofother variables.Resultsofthese analyses are summarizedin Table l. (The possibility that the
variablesmight operateas moderatorsof the age relations
was examinedby meansof cross-productinteractionterms
enteredafter the main effects in the regressionequation.
None of theseinteractionswas significant.)
It can be seen in Table I that there was relatively little
attenuationof the agerelationsaftercdntrollingthe healthand
education variables. That is, 28.1o/oof the digit-symbol
variancewas relatedto agebeforecontrollingthesevariables,
and,27.3o/owas still relatedto age after both variableswere
controlled. In contrastto the situation with the health and
educationvariables,therewas substantialreductionofthe age
relationsafter controlling the perceptualcomparisonspeed
variable. The 0.97o of the variance related to age after
removingthe varianceassociated
with perceptualcomparison
speedwas still statistically
signi{icant,but it is clearlyquite
small. It is also noteworthythat therewas moderateattenuation of the age-relatedvarianceby control of the working
memoryvariable,but it was smallerthanthat associated
with
perceptualcomparisonspeed,and therewas little it could add
to the further attenuationof the age relations above that
attributableto perceptualcomparisonspeed.
Three major conclusionsseemwarrantedfrom the results
of this study. The first is that the large negativerelations
betweenageanddigit-symbolperformancearecharacterized
95
85
5
I
o
O A <
o " U)
o 5 5
; 4 5
(t)
'6)
35
i5
25
15
5
I
I
I
I
20s
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
50s
I
I
I
I
I
I
I
I
T
I
I
I
T
lr
Age Decade
Figure l. Distribution of WAIS-R Digit Symbol Substitutionscoresby
decade.Between 129 and 169 adultsare reDresented
in eachdecade.
Pt24
SALTHOUSE
Table 1. Age-RelatedVariancein Digit-Symbol Performance
Before and After StatisticalControl of Other
Variables,Study l (N : 910)
Cumulative
R2
Increment
inR2
F for
increment
Age
.287
.28'7
3 6 9l.3 x
Health
Age
.025
.295
.025
.270
3t.6'7+
348.42*
Education
Age
.o43
.329
.043
.286
57.01*
388.20x
Health
Education
Age
.025
.060
.333
.025
.035
.273
33 . 4 9 x
41.96*
368.79x
Health
Education
Perceptualcomparison
speed
Age
.025
.060
.025
.035
50.64x
72.52*
characteristicof the digit-symboltask that is contributingto
the large negativerelations betweenage and digit-symbol
performanceis also presentin the perceptualcomparison
tasks. Becausethe perceptualcomparisontasks are superficially quite distinct from the digit-symboltask, this finding
provides convergentevidence for the hypothesisthat age
differencesin the digit-symboltestare a reflectionof an agerelatedslowingof many processingoperations.The next two
studieswere designedto provide discriminantevidencein
the form of resultsinconsistentwith intemretationsbasedon
factorsrelatedto writing speedor r.roiy limitations.
Study 2
Although the resultsof Study I appearconvincing in
indicatingthat many of the age-relateddifferencesin digitsymbol performanceare associatedwith the sameprocesses
.550
.490
1001.62+
responsiblefor agedifferencesin simpleperceptualcompar.559
.009
I 9.20*
ison tasks,the analysesare limited by the coarseness
of the
Health
.02s
.025
38.24*
measure of number of correct substitutions. A primary
Education
.060
.035
54.75*
motivation for this study was the assumptionthat more
Working memory
. 3l 9
.259
399.I 5*
detailed
examinationof the processesinvolved in perforAge
.4t6
.097
I5 0 . 6 7 *
manceof the digit-symboltaskmight be possibleby impleHealth
.025
.025
52.62*
mentingthe task on a computer.That is, becausethe comEducation
.060
.035
7 5 . 3 *5
puter
can preservecompleteinformationabout responsesto
Perceptualcomparison
individual
digit-symbolpairs, the data can be analyzedto
speed
550
.490
1040.74*
determinethe relationsbetweenmedian responsetime and
Working memory
572
.o22
46.91*
the serial position of the targetdigit in the code table. This
Age
516
.004
9.22*
information can, in turn, be used as an indication of the
* p< . o l .
searchstrategyusedby the individual, or his or her reliance
on memory of the digit-symbol pairs while performing the
task. As an example,if young adultshave less need than
by a shift in the entire distribution, and do not appearto
older adults to refer to the code table to retrieve the digitreflecta tendencyfor someolder adultsto havedramatically
symbol pairing, then their serialpositionfunctionsmight be
lower scores while others continue to perform like the
expectedto be flatter than thoseof older adults.
majority of young adults.Although thereare somerelatively
Becausedataare to be collectedfrom both the paper-andhigh-performing older adults, there are also a number of
pencil and computerversionsof the digit-symbol task, it is
low-performingyoung adults,and the age-relatedreduction
also of interestto examine the relations between the two
in the meanlevel of performancewas not associatedwith an
performancemeasures.Both the correlationandthe ratiosof
increasein the varianceof the distribution.
the time per responsecan be investigated.The correlation
The secondconclusionis that the age relations in this
indicatesthe extentto which the two measuresreflect comsample are largely independentof self-reportedhealth status
mon processes,andthe ratio is informativeaboutthe relative
and amountof education.This doesnot meanthat the factors
time required to write symbols (in the paper-and-pencil
of health and educationhave no effect on the age trendsin
version)as opposedto merely pressingone of two keys (in
digitsymbol performancebecausethe rangeof variation in
the computerversion).If the peripheral-motorhypothesisis
the health and education measures was rather restricted.
correct,thenolder adultsshouldhavemuch smallerratiosof
However,asthe valuesin this samplewereat the high end of
scoresin the computerversion to scoresin the paper-andthe scales,it can be infened that the substantialnegative
pencil version than young adults becausea greaterproporrelations betweenage and digit-symbol performanceare not
tion of their responsetime is devotedto writing the symbols.
an artifact of age differencesin health or education,because
thoserelationsareevidentevenamonghighly educatedadults
METHoD
who report themselvesto be in good to excellent health.
The third major conclusionfrom the resultsof this studyis
Subjects.- Datawere availablefrom a total of 135young
that there is considerablecommonality in the age-sensitive adults and 80 older adults. Young adults were college
aspectsofthe digit-symboltest and the age-sensitive
aspects studentswho receivedextracredit in a psychologycoursefor
of the perceptualcomparisontasks.That is, statisticalcontheir participation.Older adultswererecruitedfrom newspatrol of the composite measureof perceptualcomparison per advertisements
andreferralsfrom otherparticipants.The
speedresultedin a 96.l%oattenuation(i.e., from 27.3 to
age range of the young adults was l1 to 28 years, with a
0.9Vo)of the age-relatedvariancein Digit Symbol Substitumean of 19.7, and that for the older adults was 51 to 80
tion Test performance.Theseresultssuggestthat whatever
years, with a mean of 64.5. Forty-six percentof the young
Pt25
DIGIT SYMBOL
adults were women, as were 55Vaof the older adults. The
averageyearsofeducation were 13.6 (SD : 1.3) for young
adultsand 15.7 (SD : 2.2) for older adults,and the mean
self-assessed
healthratings(on a scalefrom I : excellentto
:
5 poor) were 1.5 (SD : 0.7) for young adultsand I .8 (SD
: 0.9) for olderadults.
Procedure. - The WAIS-R Digit Symbol Substitution
Test and a computer-controlledversion of this test were
administered,in this order, to the individuals before they
participatedin one of severalexperimentsconcernedwith
working memory.The computerversionof the testconsisted
of a displayof the standarddigit-symbolcodetableat the top
of the screen,and a display of a single digit-symbol pair
centeredin the middle of the screen. The code table remained constant across trials but the digirsymbol pair
changedfrom trial to trial. On one-halfof thetrials, the digitsymbol pair was correct, in that it conespondedto the
associationrepresentedin the code table, and on one-halfof
the trials it was incorrect.The task for the participantwas to
classifythe pairs as CORRECT or INCORRECT as rapidly
"/"
as possible by pressingthe
key on the keyboard for
"Z" key
CORRECT, and by pressingthe
on the keyboard
for INCORRECT. A practice sequenceof 20 trials was
presentedfollowed by the experimentalsequenceof 90 trials
(consistingof a randomarrangementof 5 CORRECT and 5
INCORRECTtrialsfor eachof the 9 disits).
RESULTS
ANDDISCUSSIoN
The age differences in performance on the paper-andpencil digit-symboltask in this samplewere similar to those
in other studiesas the age correlationwas -.61, with a
regressionslope of - .41 items per year. Median time per
responsein thecomputerversionhada correlationwith ageof
.75, and a correlationof - .73 with the scoreon the standard
Digit Symbol SubstitutionTest. Accuracy in the computer
versionwas very similarin the two groups(i.e.,94.9Vofor
young adults and 95.97ofor older adults),and thus was not
analyzedfurther. Neither health,education,nor genderwas
responsiblefor significantmediation(attenuation)
or moderation (interaction)ofthe agerelations,andthusthesevariables
were also ignoredin subsequent
analyses.
Two multiple regressionanalyseswere conductedto estimate the amount of age-relatedvariance in the standard
paper-and-pencil
scores.The initial analysis,with ageas the
only predictor,yieldedan R2 for age of .316, F(l,2lO) :
126.30,p < .01. In the secondanalysis,themediantime per
responsein the computer version of the digit-symbol task
was enteredas a predictorbeforeage.This analysisrevealed
with an incrementin R' of .009,
that agewas only associated
F(1,209) : 3.90, p < .05. As in the previous study,
therefore,it appearsthat most ofthe age-relatedvariancein
digirsymbol performanceis sharedwith a differenttask, and
in this particularcaseit is one that does not even sharethe
samekind of response.
Estimatesof the relative amount of time devotedto processesother than writing the symbols were derived by
test into
convertingscoreson the standardpaper-and-pencil
time per item, and thendividing thesevaluesinto the median
time per responsein the computerversionof the test. These
ratios averaged.83 for young adults and .94 for older adults
(z : 4.76, p < .01), suggestingthat the young adults
devotedapproximatelylTVoof their averageresponsetime
to writing, whereasonly 67o of the total responsetime of
older adultswas attributableto processesrelatedto writing.
However, interpretationof this result is complicatedby a
confoundingof task versionwith order of presentation.That
is, becausethe computerversionwas alwaysperformedafter
the paper-and-pencilversion, the differencein ratios could
reflectgreaterlearningfrom the first to the secondteston the
part of young adultsratherthan a greaterpercentageof time
devotedto writing. Unfortunately,thesetwo factors could
not be distinguishedwithin the presentdesign.It is neverthelessimportantto notethat thereis no indicationin thesedata
that the requirementto write the symbolsexertedmore of an
influenceon older adults than on young adults, and in this
respectthe results are consistentwith those of the studies
cited earlier (Erber, 1986; Erber et al., l98l; Salthouse,
1988; Storandt, 1976)in which young and old adults were
found to have similar ratios of symbol-copying to digir
symbol substitutionperformance.
Medianresponsetimes in the computerversionof the test
were analyzedaccordingto targetdigit (or serialposition in
the code table)and trial type (positiveor negative).These
data are illustratedin Figure 2, and results of the Age x .
TargetPosition x Trial type analysisof variance(ANOVA)
are summarizedin Table2.
It is evident in Figure 2 that the serial position functions
for responseson positive trials were similar for young and
old adults.In both groups,the patternis for responsetimesto
be somewhatfaster than averagefor digits at the extreme
positions(i.e., l and9) andfor the digit 6. The advantage
of
the digits at the end positions is probably attributableto a
strategyofsearchingthe codetablein an outside-inmanner,
and responsesto the digit 6 are likely fast becausethe
associatedsymbol (0) is distinctiveand easily remembered.
The most important point regarding the serial position
effects,however,is that they appearvery similar in the two
2000
6 tzso
a
; 1s0o
E
i= 1250
o
o
5 1000
Old - Negative
t.
'
)''-""i' "' z\
/
r
r
:
1
\
'
Youno - Posilive
+
\
'
r
r
!
Old - Positive
Young -l.le€ative
r
l_t
r
t
r
ct
E zso
(r
6
5UU
o
o
>
250
0
r
'
t
r
2
l
3
l
4
r
5
r
6
l
7
A
9
Digit
Figure 2. Median responsetime as a functionof targetserialpositionand
trial type for the computer digit-symbol task in Study 2. Each data point
representsthe median of an averageof 5 responsesfor 135 young adults or
80 older adults.
SAL,THOUSE
Pt26
Table2. ANOVA Resultsfrom Studies2 and3
Study3
Study2
Digit-Symbol
Age (young vs. old)
F
df
MSe
DigirSymbol
Digit-Digit
27.36*
l,78
2295
26.26*
l,78
35.53x
8,1688
78
12.'70*
8,624
88
| -94
8,624
l8
2.38
8,1688
78
0.82
8,624
88
t.t2
8,624
l8
79.27*
1,78
148
101.22*
1,78
35
15.56*
l,2ll
107
0.08
1,78
148
1.69
1,78
35
ti.67*
8,1688
8l
2.24
8,624
90
1.90
8.624
l6
I.83
8,624
90
0.34
8,624
l6
206.03x
l,2tl
l 738
o-t /
Position(of targetdigit in code table)
df
MSe
Age x position
F
df
MSe
Trial type (positivevs. negative)
289.90*
F
l,2ll
df
l0'7
MSe
Age x trial type
df
MSe
Position x trial type
F
df
MSe
Age x position x trial type
F
df
MSe
2.63*
8 ,16 8 8
8l
* p< . o l .
groups, and the Age x Position interaction was not
significant.
were generallyslower than positive
Negativeresponses
responses,andthe absolutedifferencebetweenthe two types
oftrials was largerfor older adultsthan for young adults,as
reflectedby the significantAge x Trial type interaction.It
shouldbe noted, however,that the trial type differencewas
very similar in the two groups when expressedin proportional terms. That is, ratios of times for negativetrials to
times for positivetrials averaged1. l3 for young adultsand
L 14 for olderadults(z : - .55).
The major findings of this study were that the computercontrolled version of the Digit Symbol SubstitutionTest
yielded measuresexhibiting a similar magnitudeof relation
to chronologicalage as the standardversion, and also provided informationrelevantto the strategiesusedto perform
the task. The lack of age differencesin the serial position
pattemsand the constantproportionof negativeto positive
responsetimes in the two groupssuggestthat young and old
adultsperformedthe task in a similar manner.
Study 3
The serialpositioneffectsin Study2 wereinterpretedas a
reflectionof the strategyusedby the researchparticipantto
searchthe code table for the correct digifsymbol pairing.
However, becausethe order of the digits in the code table
it is possiblethat the
was fixed (i.e., in a I to 9 sequence),
serial position effects actually representdifferencesin responsetimesto specificdigits. This alternativeinterpretation
can be investigatedby examining the serial position functions in a version of the task in which no inspectionof the
code table is required. If the serial position effects are
attributableto variationsassociatedwith specificdigits, and
not to the position of the digit within the code table, then
thoseeffectsshouldstill be evident in this modified version
of the task. In contrast,flat serial position functions would
be expectedin this task if they originate becauseof the
mannerin which the code table is searched.
Two computer-controlledtasks were thereforeadministeredin this study- the digit-symboltask of Study 2 and a
new digit-digittask.The digit-digittaskwas identicalto the
digit-symbol task, except that no symbols were presented
and, thus, there was no need to refer to the code table to
evaluatethe correctnessof the pair.
The availability of performancemeasuresfrom similar
tasks diff'ering in the need to rely on the code table also
provides an opportunity to examine the role of memory
That is, the magniprocesses
in digit-symbolperfbrmance.
tude of the correlationbetweenthe digit-symbol and digitdigit measuresshould indicatethe extent to which they
reflect common processes,and the ratio of digit-digit to
digit-symbol performanceshould be informative about the
relativetime neededto refer to the code table.
METHoD
Subjects.-A totalof l3l adultsbetween2l and80years
of age (mean age : 46.5 years,SD : 15.6)contributed
data to this study. All researchparticipantswere males
recruitedfrom lettersand other contactsto groupspresumed
usingspatialvisuto havemoderateto extensiveexperience
alizationabilities.The averagenumberofyearsof education
health
was 16.2(SD : 2. l), and the averageself-assessed
r a t i n gw a s 1 . 4( S D : 0 . 6 ) .
Procedure. - The two tasks reported here were performed immediatelyafter participatingin a study concerned
with influencesof age and experienceon spatial visualization abilities. The digit-symbol task was identical to the
computer-controlledtask describedin Study 2. The digitdigit task had a similar format but no symbols were displayed.That is, the codetableconsisted
ofrepetitionsofthe
digit from the top box in the bottom box, and the probe
displaysconsistedof two digits that were eitherthe same(on
50Voof the trials) or different (on 507o of the trials). All
participants performed the digit-symbol task before the
digit-digit task.
ANDDISCUSSION
RESULTS
Median times in the digit-symbol and digirdigit tasks
averagedl.5l and 0.67 seconds,respectively.Because
accuracywas high (i.e.,94.8Voin the digirsymbol task and
97 .67oin the digirdigit task), it was not consideredfurther.
The two time measurescorrelated.61 with eachother. and
DIGIT SYMBOL
had ageconelationsof .53 for the digitsymbol measureand
.50 for the digit-digit measure.Neither healthnor education
was significantlycorrelatedwith either digirsymbol time or
digirdigit time and, thus, thesevariableswere isnored in
subsequentanalyses.
Hierarchical regressionanalysesrevealed that the age_
related variance in the digit-symbol measurewas reduc"ed
from 27.l to 6.4Eoafter controlling the varianceassociated
with the digit-digit measure.This result is similar to that of
StudyI in indicatingthat thereis considerableoverlapin the
age-relatedvariance in measuresof digit-symbol perfor_
manceand in measuresof performancein simpleperieptual
comparisontasks.
The ratio of mediantime in the digit-digit task to that in
the digit-symboltaskaveraged.43, indicatingthat processes
concernedwith determiningwhetherthe pairing of digit and
symbol was correctrequiredan averageof about Sln Li.e.,
(1.0 - .43) x l00l of the total responsetime. The correla_
tion of this ratio with agewas only .007. indicatingthat the
proportional time devoted to evaluating the digii-symbol
associationremainedquite constantfrom about age 20 to
age80.
The sample was divided into three groups, and perfor_
manceof the 38 young adults(age2l to 34, mean age :
28.2 years)and42 olderadulrs(age56 ro 80, mean : 65.3
years)contrastedin a mannersimilar to Study 2. The serial
positionfunctionsfor positiveand negativetrials in the digit_
symbol and digit-digit tasksare illustratedin Figure 3, ind
the resultsof the ANOVA are summarizedin Table 2.
Two results from Figure 3 and Table 2 are especially
.
important to note. The first is that the serial position func_
tions for the digit-symboltask closely resemblethose of
Study 2. The major differenceseemsto be that the times of
the young adultsin the currentstudyare slowerthanthoseof
theyoung adultsin Study2, perhapsbecausethe averageage
of the young adultsin this samplewas almost9 yearsgreater
than that of the young adultsin the earlier sample.
The secondinterestingaspectof the resultsin Figure 3 is
OId DS NEG
2000
o
o
@ 1750
I
3
/
I 1s00
.t -,,
"t
a
i=
o 1250
a
. l
rt
-'
I
ord DS Pos
-t..
OIdDD NEG
.,
r
'
I
c
t r
t
a
t
1
.
t"--
--r,
I
t_
-
'
./
\
otd DD Pos
\
.
.
Youno
DS NEG
: l
t---
I 1000
YounoDS POS
+Younq
- aDD NEG
o
YounoDD POS
a
c
I
a
c(U
6 500
o
250
l
1
t
_
2
L
3
t
4
t
5
L
6
L
7
8
9
Digit
Figure 3. Median responsetime as a function of target serial position and
trial type-for the computer digit-symbol (DS) and computer digit-digit (DD)
tasks in Study 3. Each data point representsthe median of an averageof 5
responsesfor 38 young adults or 42 older adults. NEC, negatiue;pOS,
Positive.
Pl27
that there was no evidenceof a serial position effect in the
responsetimes in the digit-digit task. As mentionedprevi_
ously, this finding is consistentwith the interpretatio; that
the serial position effects in the digirsymbol task reflect
processes
associatedwith searchofthe codetableandarenot
attributableto factorsspecificto particulardigits.
General Discussion
The resultsof thesestudieslead to two maior conclusions
concerningthe relationbetweenage and performanceon the
Digit Symbol SubstitutionTest. The firsi conclusion,based
on the re-sults
of Study l, is that the age trendsare produced
by a gradualshift ofthe entiredistributionofscores.This is a
somewhatsurprisingfinding, becauseit is occasionallysug_
gestedthat negativerelationsbetweenage and performanJe
on various cognitive tests are a consequenceof a small
number of people experiencingrather pionounced impair_
ments with increasedage, with many people continuing to
perform at the level of young adults (Albert, lggg; Lich_
man, 1986;Schaie,1988).The parternsevidentin Figure I
areclearly incompatiblewith this interpretation,and instead
are more consistentwith a view that the age_related
decline
in digirsymbol performanceis normativeor typical of most
individuals.Of course,longitudinalinformationis neededto
reach definitive conclusionsabout patterns of individual
aging, but the discoverythat the age-relateddeclinesin this
testarecharacterizedby a gradualrepositioningofthe entire
distribution,with little or no increasein interindividual
variability, indicatesthat the negativerelationsare not sim_
ply attributableto the presenceof a few very low scoreson
the part of a small numberof older adults.
The secondmajor conclusionis that the agedifferencesin
digirsymbol performanceprimarily seemtJreflect a slower
rateof processinginformation,and not deficitsin memoryor
in the efficiency of specific processes.Two sets of results
lead to this conclusion.One set consistsof the findinssin
Studies I and 3 that a large proportion of the age-reLted
variancein digit-symbol performanceis sharedwith mea_
sures of simple perceptualcomparison speed. Of related
interestare the high conelations betweenthe digirsymbol
andperceptualcomparisonspeedmeasuresin Study t (i.e., r
:
betweenthe paper-and-penciland computerdigit
7ql,
symbolmeasures
in Study2 (i.e., r = - .73), andbetween
the comprrterdigirsymbol and computer digit-digit mea_
suresin Study 3 (i.e., r : .61). [In order to place these
values in proper context, it should be noted that Wechsler
(1981) reportedrhat the test-retestreliability coefficienrfor
the digit-symbolscoreis . 82.I Becausethe cbmmonelement
in the paper-and-pencildigit-symbol substitution, letter
comparison, pattern comparison, computer digit-symbol,
and computer digit-digit tests seemsto be the speedwith
which elementaryoperationscan be executed. it appears
reasonableto infer that a major factor contributing to ttre age
sensitivity of digit-symbol performance is the speed of
carrying out simple processingoperations.
The other setof resultssupportingthe conclusionthat age
differencesin digirsymbol performanceare largely attribul_
able to reductions in the speed of processingare those
concernedwith the influenceof searchor decisionprocesses,
P128
SALTHOUSE
memory processes,and factors related to writing speedor
manualdexterity.The resultsof Studies2 and3 , andthoseof
the other studiescited earlier, are remarkablyconsistentin
makethe samerelativecontriindicatingthattheseprocesses
bution to the performanceof young andold adults.Estimates
of the magnitudeof the effectsof manipulationsdesignedto
investigatethesefactorsare sometimesgreateramongolder
adults in absoluteterms, but the relative contribution has
generally been found to remain constantacross adults of
differentages.This is preciselythe patternone would expect
if increasedageis associatedwith an approximatelyproporoperations
tional slowingof many elementaryprocessing
It was mentionedin the introductionthat the Digit Symbol
Substitution Test may have special significancebecause
scoreson this test are stronglyrelatedboth to chronological
age and to a variety of other measuresof cognition and
intelligence.If, as the results of these studies seem to
indicate,it reallyis the casethatthe agerelationson this test
reflect a slowing in the rate of performingbasicoperations.
then a worthy goal for future researchis to determine
whethera similar mechanismis responsiblefor the relations
betweenmeasuresof cognitivefunctioningand both ageand
digirsymbol performance.
AcKNOwLtDGt\4ENTS
This study was supportedby National Instituteon Aging Grant AG06826.
t<l Dr. Timothy A. Salthouse.School of PsyAddresscorrespondence
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ny
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ReteivedSeptamber26, 1990
AcceptedJune 6, l99l