Incremental Validity of the Successive Level Approach to Intelligence Test Interpretation.

This study examined the incremental validity of the successive-level approach to intelligence test interpretation in school psychology, focusing on the Woodcock-Johnson IV (WJ IV) Tests of Cognitive Abilities and their prediction of academic achievement and oral language outcomes. The successive-level approach assumes that unique, meaningful information is captured at multiple levels of intelligence tests (e.g., full scale, index-level, subtest-level), but findings showed that while the Cattell-Horn-Carroll (CHC) cluster scores contributed some unique variance beyond the General Intellectual Ability (GIA) composite, their incremental validity was generally limited and often overshadowed by the GIA. Results across normative samples aged 6 to 19 indicated that interpreting lower-level scores may lead to redundancy, supporting concerns about the duplication fallacy in clinical practice. Although some moderate to large incremental effects were observed for specific outcomes like Oral Expression, Listening Comprehension, Reading Fluency, and Math Problem Solving, the overall evidence suggests that primary interpretive emphasis on the global composite score remains warranted. The study highlights the need for clinicians to carefully consider the psychometric evidence regarding unique variance before relying heavily on lower-level scores in intelligence test interpretation.