As can be seen in , the neurologically compromised participants evidenced verbal memory deficits in comparison to the healthy participants as expected. Correlational analyses were used to determine the degree that the proposed encoding indices were related to each other and whether they might be confounded by inattention. In the participants that completed WAIS–R Digit Span (n
= 62), the ISDA encoding index was highly related to CVLT total learning (r
= −.96, p
< .001) and moderately associated with learning slope (r
= .33, p
< .05). However, only CVLT total learning was significantly associated with WAIS–R Digit Span (r
= .29, p
< .05); neither learning slope nor the ISDA encoding deficit index (r
= −.21) was correlated with Digit Span (r
= −.14). To determine whether these indices behaved differentially as a function of inattention, as hypothesized, we reran the analysis in a subgroup of participants (n
= 53)—45 neurologically compromised and 8 healthy; 72% male; 20 Caucasians, 28 African Americans, 2 Mexican Americans, 1 Asian American, 1 Native American, 1 “other;” mean age = 39.79 years (SD
= 7.21); mean years of education = 13.68 (SD
= 1.82)—that exhibited inattention on WAIS–R Digit Span (T
< 40; see Carey et al., 2004
). As can be seen in , the pattern of associations is very different for participants with attention deficits. The ISDA encoding deficit index did not correlate with either of the traditional indices (r
s ≤ −.17) or Digit Span (r
= −.10). However, CVLT total learning was correlated with Digit Span (r
= .33) and learning slope (r
=.30); the relationship between learning slope and Digit Span approached significance (r
= −.25, p
= .07).With regard to the other memory process index correlations (see ), numerous significant intercorrelations were found. However, it should be noted that forgetting and recognition–recall discrepancies were moderately associated (r
= −.45), while the ISDA consolidation deficit index was unrelated to the ISDA retrieval deficit index (r
= .03). Also, the ISDA retrieval deficit index was not associated with the recognition–recall discrepancies index (r
= .03). Additionally, although total learning and the ISDA encoding index were highly correlated in the sample as a whole, these indices were shown to be functionally distinct in a group of inattentive participants (see ).
Correlations between Digit Span and indices of encoding in inattentive participants
Correlations between memory process indices
The ISDA encoding index provides dichotomous outcomes for each list item (recalled vs. not recalled), so its internal consistency was evaluated with the KR-20
statistic. In contrast, the acquisition-adjusted consolidation and retrieval deficit indices, which are adjusted for the number of items gained during learning, are not composed of dichotomous values, so they were evaluated with coefficient alpha. While high internal consistency coefficients (> .80) are routinely expected of indices used to make important decisions about level of impairment (Anastasi, 1988
; Urbina, 2004
), indices that are used to characterize these impairments are not judged by the same standard since they are descriptive (not diagnostic) in nature (Kline, 2005
). The internal consistency estimates of such “process” indices are often decreased by two factors: a small number of items and heterogeneity in item responses due to individual differences in test-taking approach (Cronbach, 1984
; Kehoe, 1995
; Pedhazur & Schmelkin, 1991
). Heterogeneity in item responses is indeed an issue in memory test performances, as common individual differences in list-learning approaches (levels of self-initiated rehearsal, intensity of study) differentially impact immediate- and delayed-recall patterns (e.g., Geiselman, Woodward, & Beatty, 1982
). It has been suggested that moderate internal consistency estimates (.50–.70) are acceptable for experimental measures and/or descriptive indices comprised of a relative small number of items (Kehoe, 1995
; Kline, 2005
As can be seen in , ISDA indices of encoding, consolidation, and retrieval deficits showed acceptable internal consistency (.58–.77). In order to obtain internal consistency estimates that were less influenced by individual differences, we analyzed data from a subset of 67 participants that utilized a consistent list-learning strategy (indicated by a z-score of ≤1 on the CVLT’s semantic or serial clustering indices). This led to higher internal consistency estimates (.64–.84; see ). Finally, to determine whether the number of items for each of the indices was impacting their internal consistency, we artificially doubled the responses for each index and calculated internal consistency coefficients for the whole sample as well as the subset that indicated a consistent learning strategy. This analysis showed that the number of items did influence our internal consistency estimates as the coefficients increased dramatically (.80–.89 and .82–.92, respectively; see ).
Internal consistency coefficients for the ISDA as applied to the CVLT
Linear discriminant function analysis was employed to determine how well the ISDA indices could predict neurological status. The ISDA indices significantly discriminated between neurologically compromised and healthy participants: Wilks’s λ= 0.33, χ3(3) = 143.45, p < .001; canonical r(cr) =.82, η2 = .67. The encoding deficit index was the main contributor to this discriminant function (cr = .71), followed by the retrieval deficit index (cr = .54) and the consolidation deficit index (cr = .45). Overall, the ISDA indices correctly classified 93.2% of the participants as either neurologically compromised or intact, with 92.4% sensitivity and 94.3% specificity.
A follow-up linear discriminant function analysis was conducted to determine how well traditional memory process indices (i.e., learning slope,2
forgetting, recognition-recall discrepancy) could predict neurological compromise. The traditional indices significantly discriminated between neurologically compromised and healthy participants: Wilks’s λ = 0.60, χ2
(3) = 64.31, p
< .001; cr
= .40. The traditional consolidation index (forgetting) was the main contributor to this discriminant function (cr
= .83), followed by the encoding index (learning slope; cr
=.48) and the retrieval index (recognition-recall discrepancy; cr
= −.13). Overall, the traditional memory process indices correctly classified 82.2% of the participants as either neurologically compromised or intact, with 90.6% sensitivity and 78.9% specificity.
Discriminant function analysis was conducted on a subsample of participants (n
= 51)—20 neurologically compromised and 31 healthy; 82% male; 44 Caucasians, 1 African American, 6 “other;” mean age = 34.45 years (SD
= 9.39); mean years of education =14.29 (SD
= 2.18)—that had completed the Logical Memory subtest of the WMS-III, a measure of verbal learning and memory. Impaired performance on Logical Memory was determined by converting norm-derived scaled scores for total story units recalled in Logical Memory I and Logical Memory II to T
-scores and then transforming these to deficit scores (see Carey et al., 2004
). Performances resulting in a deficit score of 1 or greater (i.e., T
< 40) on either Logical Memory I or Logical Memory II were considered to reflect impaired verbal memory. The ISDA indices significantly predicted impaired verbal memory on Logical Memory: Wilks’s λ = 0.65, χ2
(3) = 26.41, p
< .001; cr
= .59, η2
= .35. This function was primarily driven by the consolidation deficit index (cr
=.89), while the encoding (cr
=.14) and retrieval deficit (cr
=.20) indices played a minor role. It should be noted that the chief role of the consolidation deficit index in this function is not particularly surprising, given that meaningful prose provides a good deal of encoding and retrieval support (e.g., Jefferies, Lambon Ralph, & Baddeley, 2004
). This discriminant function correctly classified 82.4% of the Logical Memory performances, with 74.0% sensitivity and 89.3% specificity.
An additional discriminant function analysis was carried out to determine how well traditional memory process indices (i.e., learning slope, forgetting, recognition-recall discrepancy) predicted performance deficits on the Logical Memory subtest. The traditional indices also predicted Logical Memory deficits: Wilks’s λ = 0.79, χ2(3) = 11.25, p = .01, cr =.46, η2 = .21. This function was primarily driven by forgetting (cr =.67), followed by recognition-recall discrepancies (cr = −.53) and learning slope (cr = −.11). This discriminant function correctly classified 72.5% of the Logical Memory performances, with 93.0% sensitivity and 48.0% specificity.
Given the satisfactory psychometric findings regarding the ISDA as applied to the CVLT, we further analyzed the healthy participants’ (n = 53) data in order to derive useful normative references for future research. It should be noted that we used acquisition-corrected indices of consolidation and retrieval deficits for these analyses. Correlational analyses among age, education, and the ISDA indices indicated a positive association between age and the encoding deficit index (r = .48, p < .001). Neither the consolidation deficit nor the retrieval deficit indices were related to age (r =.15, r = .07, respectively). None of the ISDA indices was associated with years of education (rs = −.09 to −.11).
To further explore the relationship between age and the encoding deficit index, we stratified the healthy sample into three age groups (18–28 years, n = 12; 29–39 years, n = 19; 40–50 years, n = 22) and conducted an Age Group × Encoding Deficit Index ANOVA. The ANOVA revealed a significant main effect of age, F(2, 50) = 8.40, p = 0.001, ηp2 = .25. Tukey post hoc tests indicated that 40–50-year-olds had greater encoding deficits than the other two younger age groups. The 18–29- and the 29–39-year-olds did not differ with respect to the ISDA encoding deficit index. Based on these results, we present preliminary normative data for the ISDA indices stratified by two age groups (18–39 and 40–50 years) in .
Age corrected experimental normative data for the ISDA as applied to the CVLT