Does WISC-V scatter matter when it comes to academic achievement in neurodevelopmental disorders? It depends

The interpretability of FSIQ is often believed to rely on variability across index scores, although studies show that FSIQ is the strongest predictor of academic achievement, regardless of scatter. This study investigated whether IQ scatter affects the predictive validity of FSIQ on academic achievement in youth with neurodevelopmental disorders. A retrospective chart review was conducted with patients aged 6-16 evaluated at a developmental assessment clinic between 3/2018 and 12/2022 (N = 76, 74% male, 76% White). Moderation analyses were conducted using the FSIQ as a predictor, index scatter as a moderator, and academic achievement factor as a dependent variable. Index scatter was a significant moderator for the predictive effect of FSIQ on broad achievement and broad written language. The Johnson-Neyman technique revealed that the effect of FSIQ on broad achievement was significant when the index scatter was ≤ 54 in our sample. The results are consistent with previous studies, as the robust predictive validity of FSIQ was overall demonstrated. However, this study also provides novel information suggesting that the predictive validity of the FSIQ for academic achievement in neurodevelopmental disorders may weaken as index score scatter increases, a relationship that became statistically nonsignificant within our sample at highly elevated levels.