Incremental Validity of the Successive Level Approach to Intelligence Test Interpretation

This study investigated the incremental validity of the successive-level approach to intelligence test interpretation, specifically within the context of school psychology. The successive-level approach assumes that unique information is captured at various levels of intelligence tests (e.g., full scale, index-level, subtest-level). However, previous research has indicated that lower-level scores often fail to explain significant or meaningful variance in achievement outcomes beyond what is accounted for by the global score, suggesting potential redundancy in interpreting lower-level scores. Using the Woodcock-Johnson IV (WJ IV) standardization sample, this study examined the relationship between cognitive ability scores (GIA and CHC clusters) and achievement outcomes. The results indicated that, although the CHC subscale scores contributed some unique variance in achievement, their incremental validity was limited and frequently overshadowed by the GIA composite. These findings align with previous research, suggesting that clinicians may unwittingly commit a duplication fallacy when relying on successive-level score interpretation or related guidance in clinical practice.