A limitation of norm-based ability test scores is that they can only be used to evaluate relative change (compared with change in the norm sample), as opposed to absolute (raw) change in performance from Time 1 to Time 2. To address this limitation, a novel method (Projected Retained Ability Score [PRAS]) was developed to characterize absolute change in norm-based ability test scores. The PRAS method was applied to Differential Ability Scales®-Second Edition (DAS-II) General Conceptual Ability (GCA) scores in three cases of children with the neurodegenerative condition mucopolysaccharidosis type II (MPS II) who were assessed at two visits, 16 to 23 months apart. Although all three cases showed declines in norm-based GCA scores, the PRAS method revealed differences in absolute change in performance. The PRAS method allows for differentiation of slower-than-average improvement or stabilization versus deterioration of cognitive ability when norm-based scores decline from Time 1 to Time 2.