Speed and memory in the eWAIS-III Digit Symbol - Coding subtest across the adult lifespan

The primary role of speed in determining Digit Symbol scores is well established. Among the important questions that remain to be resolved are: (1) whether speed accounts for all of the age-related decline in Digit Symbol scores, and (2) whether memory ability makes any significant contribution to Digit Symbol performance, especially after controlling for speed. We analyzed data from the WAIS-III/WMS-III standardization sample to resolve these issues. As expected, speed (Digit Symbol-Copy) correlated very strongly with Digit Symbol-Coding. Memory (Digit Symbol-Incidental Learning or WMS-III index scores) correlated more moderately with Digit Symbol-Coding. Even after controlling for variance in Coding explained by Copying, a statistically significant proportion of the residual variance was explicable in terms of memory functions. The contribution of memory to Digit Symbol-Coding, while relatively small, is real. In addition, a small portion of the age-associated decline in Coding scores cannot be accounted for by Copying scores. (C) 2003 National Academy of Neuropsychology. Published by Elsevier Ltd. All rights reserved.