What do adult age differences in the Digit Symbol Substitution Test reflect?

Results from three studies are reported in which adults between 18 and 84 years of age performed various versions of the Digit Symbol Substitution Test. The first study revealed that the age-related declines in digit-symbol performance were largely independent of both the amount of education the participants had received and their self-reported health status, and were characterized 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 measure of perceptual comparison speed, however, implying considerable commonalities between perceptual comparison speed and 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 responses and searching the code table.