Forward and backward memory span should not be combined for clinical analysis.

The practice of combining forward and backward memory span, as represented so prominently on the various Wechsler Scales, to arrive at a composite score for clinical interpretation is examined historically and actuarially using a large (N = 1,342) nationally stratified random sample of children from ages 5 years through 19 years. Past literature does not support the additive nature of forward and backward memory span as elements of a common process. Factor analyses of forward and backward recall using both digits and letters indicate that the two memory processes are distinct as well and should not be combined for clinical interpretation.