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Psychology and Aging 2004, Vol. 19, No. 1, 211–214

Copyright 2004 by the American Psychological Association, Inc. 0882-7974/04/$12.00 DOI: 10.1037/0882-7974.19.1.211

Adult Age and Digit Symbol Substitution Performance: A Meta-Analysis William J. Hoyer, Robert S. Stawski, Christina Wasylyshyn, and Paul Verhaeghen Syracuse University This article reports the results of a meta-analysis of the effects of age, education, and estimated year of measurement on scores from the Wechsler Adult Intelligence Scale and the Wechsler Adult Intelligence Scale—Revised Digit Symbol Substitution Test. Analysis of effect sizes for age reported in 141 studies published between 1986 and 2002 indicated a mean standardized difference of –2.07. Age accounted for 86% of the variance in a regression model using age, education, and year submitted as predictors of Digit Symbol scores. There was no association between years of education or year submitted and Digit Symbol scores for younger adults or older adults.

provide an ideal base for a comprehensive meta-analysis of the relations between age and DSST scores and for exploring possible relations between DSST scores and years of education and years of measurement (estimated using year submitted for published manuscripts). In regard to possible relations between DSST scores and educational level, the available data, all from nonaggregated data sets, suggest no relationship (Birren & Morrison, 1961; Salthouse, 1992). It would be useful to show that age–DSST relations are independent of years of formal education across a broad range of study samples. Although there are no reports directly examining the relationship between age-related declines in DSST scores and years of measurement, Schaie (1994) reported relatively stable performance for measures of perceptual speed for cohorts born between 1907 and 1966 and tested between 1956 and 1991. However, this finding stands somewhat in contrast to analyses by Flynn (1987) and Raven (2000) indicating secular increases in intelligence test scores. Flynn reported gains of between 5 and 25 points in IQ scores during the past 50 years in secondary analyses of large data sets taken from different countries. Raven described substantial secular increases in scores on the Raven’s Progressive Matrices (RPM) during the past 20 years, although RPM scores taken prior to 1979 were found to be stable across testing occasions. In light of the generally strong relations between intelligence and perceptual speed reported in the literature (e.g., Postuma, de Geus, & Boomsma, 2001; Salthouse, 1996; Sliwinski & Buschke, 1999; Verhaeghen & Salthouse, 1997; Zimprich & Martin, 2002), it would be useful to know whether there have been secular increases in DSST scores in recent years. In this study, we report the results of a meta-analysis of the effects of age, education, and year of measurement (estimated from year submitted for published manuscripts) on DSST performance using all pertinent data published in Psychology and Aging and the Journals of Gerontology (all subsections) between 1986 and 2002. That there is a large number of studies reporting DSST scores for younger and older adults and that the format of the WAIS and WAIS–R versions of the DSST has remained unchanged (until recently; see Wechsler, 1997) makes it an ideal dependent variable for assessing age–performance relations as a function of educational characteristics of the samples and year submitted.

One of the most widely used instruments for describing the performance of younger and older adults in cognitive aging studies is the Digit Symbol Substitution Test (DSST) from the Wechsler Adult Intelligence Scale (WAIS; Wechsler, 1955) and the Wechsler Adult Intelligence Scale—Revised (WAIS–R; Wechsler, 1981). The DSST has two properties that make it a valuable tool for aging research. First, the DSST seems to serve as a robust marker for describing sample characteristics in studies of age differences. Age–DSST correlations of between –.46 and –.77 are typically reported (e.g., Birren, 1965; Birren & Morrison, 1961; Doppelt & Wallace, 1955; Kaufman, Reynolds, & McLean, 1989; Royer, Gilmore, & Gruhn, 1981; Salthouse, 1992). Second, scores on the DSST have been shown to exhibit strong correlations with measures that in some way involve perceptual speed (Laux & Lane, 1985; Lindenberger & Baltes, 1997; Salthouse, 2000; Sliwinski & Buschke, 1999), and perceptual speed or processing speed is known to be a large source of the variance in many age–performance relations (e.g., Birren, 1965; Cerella, 1990; Madden, 2001; Salthouse, 1996). The DSST consists of a look-up table showing pairs of digits and hieroglyphic-like symbols and rows of boxes with a digit in the top section and an empty space in the bottom section of each box. A participant’s score is the number of empty boxes completed in 90 s. The DSST is easily administered, and the procedures for administration and scoring of the test leave relatively little room for variation. Because the format and test materials for the DSST have remained unchanged (Wechsler, 1955, 1981), the DSST data already available in studies published during the past 16 years

William J. Hoyer, Robert S. Stawski, Christina Wasylyshyn, and Paul Verhaeghen, Department of Psychology and Center for Health and Behavior, Syracuse University. This research was supported by National Institute on Aging Grant AG-11451. We thank Serge Onyper and Silvie Semenec for assistance with a preliminary data collection and Ulman Lindenberger for comments on an earlier draft. An Excel spreadsheet listing the studies providing data for the metaanalysis and the effect sizes for each of the samples is available from William J. Hoyer. Correspondence concerning this article should be addressed to William J. Hoyer, Department of Psychology, 430 Huntington Hall, Syracuse University, Syracuse, NY 13244-2340. E-mail: [email protected] 211

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212

Table 1 Means and Ranges for Measures of the Characteristics of the Research Participants Younger adults

Older adults

Measure

No. of studies

M

Range

M

Range

Age (years, unweighted) Age (years, weighted by n) Education (unweighted) Education (weighted by n) Digit symbol (unweighted) Digit symbol (weighted by n)

139 139 117 117 138 138

21.4 21.6 13.9 14.1 69.3 69.8

18.1–30.7

69.5 69.8 15.2 15.3 48.2 48.6

60.7–78.9

12.0–16.7 51.2–82.7

11.7–17.9 38.8–66.8

Note. The age groups consisted of 3,731 younger adults and 3,876 older adults. Education ⫽ number of years of formal education. Digit symbol ⫽ score on Wechsler Adult Intelligence Scale or Wechsler Adult Intelligence Scale—Revised Digit Symbol Substitution subtest.

Method Sample of Studies All volumes of Psychology and Aging and the Journals of Gerontology (all subsections)1 published between 1986 and 2002 were hand searched for articles reporting mean raw scores for the WAIS or WAIS–R DSST (the standard paper-and-pencil version). Articles reporting at least one sample of younger adults (with a mean age below 30 years) and one sample of “healthy” older adults (with a mean age above 60 years) were included. Scores derived from samples with participants reported to have dementia were excluded. For longitudinal studies and for studies reporting multiple administrations of the DSST to the same participants, only scores from the first administration of the test were included. A total of 141 studies reporting data from 3,731 younger adults and 3,876 older adults satisfied these inclusion– exclusion criteria; 99 of the studies were in Psychology and Aging, 40 were in Journal of Gerontology: Psychological and Social Sciences, and 2 were in Journal of Gerontology: Medical Sciences. Mean DSST scores and means and ranges for age and education for the younger and older samples are reported in Table 1.

Statistical Analysis Procedures A standard effect size analysis was performed to determine the age effect on DSST scores (Hedges & Olkin, 1983). For each combination of scores for younger adults and older adults, the mean standardized difference for the size of the age effect was determined. The mean standardized difference was calculated as the mean score for older adults minus the mean score for younger adults, divided by the pooled standard deviation. If means and standard deviations were not reported, inferential statistics were used to calculate the mean standardized difference, if possible; 46 of the 141 effect sizes were extracted from t or F tests. To test whether the mean standardized difference for the size of the age effect can be represented as a single value (the overall effect size), we calculated a homogeneity statistic, Qt (Hedges & Olkin, 1983, pp. 154 –156). If Qt, which is a chi-square statistic distributed with k ⫺ 1 degrees of freedom (where k equals the number of effect sizes), exceeds the critical value, further analysis of possible moderator variables is indicated.

Results Figure 1 shows mean DSST scores by age. The averaged effect size (d) for age was –2.07, the lower bound of the 95% confidence interval was –2.12, and the upper bound of the 95% confidence interval was –2.03. The overall effect size was heterogeneous, Qt(141) ⫽ 511.18. A hierarchical regression analysis (weighted by sample size) was performed on raw DSST scores, to examine

possible moderators of the heterogeneity. In the first step (Model 1), age, years of education, and year submitted for the study were entered simultaneously as predictors of DSST scores (k ⫽ 242). In the second step (Model 2), we explored whether the slopes for age, for years of education, and for year submitted differed as a function of age group. In Model 2, the three two-way interaction terms for these factors were added as predictors of DSST scores. The unstandardized B, standardized beta, and the t values for each of the predictors in the two regression models are reported in Table 2. The overall effect of Model 1, the three-parameter model, on DSST scores was significant, F(3, 238) ⫽ 446.37, MSE ⫽ 20.93, R2 ⫽ 0.85. The coefficients were significant for age ( p ⬍ .001) and years of education ( p ⫽ .019), and the coefficient for year submitted was not significant (t ⬍ 1). The overall effect of Model 2, the six-parameter model, on DSST scores was also significant, F(6, 235) ⫽ 224.72, MSE ⫽ 20.85, R2 ⫽ 0.85. The fit for Model 2 was no better than the fit for Model 1, ⌬R2 ⫽ 0.002, F(3, 235) ⫽ 1.31. In Model 2, only the coefficient for age ( p ⫽ .002) was significant. The absence of interaction terms suggested that the slopes for age and years of education for the two age groups were not different. It is important to note that the Nonsignificant Age (within groups) ⫻ Age Groups interaction (t ⫽ 1.70) suggests that the slopes of the age–DSST relation were not different between the two age groups and that the age–DSST relation is not an artifact or distortion associated with using extreme groups. The regression slope, – 0.46 items per year (Model 1), was consistent with the value of the slope of – 0.47 reported by Salthouse (1992) and the slope of – 0.43 reported by Emmerson, Dustman, Shearer, and Turner (1989) using a symbol digit test. Figure 2 shows the relation between DSST scores and education, and Figure 3 shows the relation between DSST scores and year submitted. The plots show the data from the 242 samples that were used in the regressions. The differences in the distributions of years of education between the samples of younger adults and older adults were quite striking and are shown in histogram form in Figure 4. As reported in Table 2, there was no relationship 1

This journal was called Journal of Gerontology prior to and including Volume 41 (1986). The subsections of the Journals of Gerontology included in the search were Biological Sciences, Medical Sciences, Psychological Sciences, and Social Sciences.

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Figure 1. Digit symbol scores as a function of age. Triangles indicate younger adults, and circles indicate older adults. DSST ⫽ Digit Symbol Substitution Test.

between DSST scores and years of education when differences between age groups were taken into account (Model 2). Another weighted least squares regression analysis was performed to determine whether the effect sizes for DSST scores were predicted by the size of the difference in age or education between age groups. The overall effect of the regression model on DSST scores was significant, F(2, 117) ⫽ 5.23, MSE ⫽ 15,845, R2 ⫽ 0.08. The coefficient for the age difference was significant (t ⫽ –3.18, p ⫽ .002), indicating that the strength of the age–DSST relation was related to the size of the chronological age differences for the groups. The coefficient for difference in years of education was not significant (t ⬍ 1).

Discussion The aim of this study was to assess the magnitude of the age effect in DSST scores by applying meta-analytic methods to the

213

Figure 2. Digit symbol scores as a function of number of years of education. Triangles indicate younger adults, and circles indicate older adults. The solid line indicates the regression line for younger adults, and the broken line indicates the regression line for older adults. DSST ⫽ Digit Symbol Substitution Test.

data reported in age-comparative studies published in Psychology and Aging and the Journals of Gerontology during the past 16 years. The magnitude of the effect of age on DSST scores obtained from 141 age comparisons was substantial (d ⫽ –2.07). This finding confirms the high correlations between age and DSST scores reported in standardization studies (e.g., Birren & Morrison, 1961; Kaufman et al., 1989; Wechsler, 1997) and in large nonaggregated data sets with digit symbol and symbol digit measures (e.g., Royer et al., 1981; Salthouse, 1992, 2000). Although the biobehavioral processes underlying age effects on DSST scores are not well understood (e.g., Laux & Lane, 1985; Piccinin & Rabbitt, 1999; Salthouse, 1992, 2000), the results of the present study indicate unequivocally that the speed of carrying out

Table 2 Summary Statistics for Two Regression Models Predictor

B

␤

t

⫺0.95 0.07 ⫺0.00

⫺33.09* 2.37* ⫺0.61

⫺1.34 0.09 ⫺0.01 ⫺0.10 ⫺0.73 1.20

⫺3.14* 1.79 ⫺0.18 ⫺0.27 ⫺1.47 1.70

Model 1 (R2 ⫽ .85) ⫺0.46 0.62 ⫺0.01

Age Education Year submitted

Model 2 (R2 ⫽ .85) Age Education Year submitted Age ⫻ Education Age ⫻ Year Submitted Age ⫻ Age Groups (within groups)

⫺0.65 0.82 ⫺0.01 ⫺0.15 0.01 0.40

Note. Models were least squares regressions weighted by n. Model 1 used three predictors, and Model 2 used six predictors. Data for year submitted were the years indicated in the date of submission for the manuscripts. * p ⬍ .05.

Figure 3. Digit symbol scores by year submitted. Triangles indicate younger adults, and circles indicate older adults. The solid line indicates the regression line for younger adults, and the broken line indicates the regression line for older adults. DSST ⫽ Digit Symbol Substitution Test.

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214

Figure 4. Frequency distribution of education scores for samples of younger adults (unfilled bars) and older adults (hatched bars).

the combination of coding and substitution processes that comprise DSST performance is slower for older adults than for younger adults. Furthermore, the age–DSST relation, derived here from a large and wide-ranging set of descriptive data, was independent of years of education and was invariant across years of measurement (estimated by year of submission). One implication of the robustness of the age–DSST relationship is to suggest that the DSST be used routinely as a general marker in age-comparative studies.

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Received January 8, 2003 Revision received March 31, 2003 Accepted April 7, 2003 䡲