Longitudinal comparison of alternate versions of the symbol digit modalities test: Issues of form comparability and moderating demographic variables

Abstract

A longitudinal examination of the comparability of two alternate versions of the Symbol Digit Modalities Test (SDMT) was undertaken on a large (N = 3,509), multiregional, healthy male sample. Significant differences were noted between the test scores of the two forms, both upon initial and longitudinal administration, with higher test scores generally being associated with the original version, and higher recall scores being associated with the alternate version. Due to these differences, it is recommended that comparisons between the forms be made only with standardized scores based on test-specific norms, such as those provided in the present study. It was determined that no significant practice effects were noted over the 2-year retest period, demonstrating the relatively stability of the two versions across time. Finally, the demographic characteristics of age, and estimated IQ were demonstrated to be significantly associated with test performance, emphasizing the importance of the stratification of normative data by these variables in interpreting SDMT performance.