Publication date: Jul 28, 2025

This study evaluated intra-individual variability (IIV) metrics, dispersion (IIV-D) and inconsistency (IIV-I), for distinguishing the following groups: attention-deficit/hyperactivity disorder (ADHD), non-ADHD psychological diagnoses (PD), no diagnosis (ND), and invalid neuropsychological test performance. A sample of 845 adults (ADHD = 518; PD = 170; ND = 39; invalid = 118) were administered a neuropsychological battery that yielded 12 indicator scores and five performance validity tests (PVTs). intra-individual standard deviation (ISD) and coefficient of variation (CoV) indexed IIV-D across the 12 test indicators, and Continuous Performance Test-3 Hit Reaction Time Standard Deviation (HRT SD), Variability (VAR), and HRT SD and VAR/mean Hit Reaction Time (HRT; e. g. HRT SD/HRT, VAR/HRT) scores indexed IIV-I. ISD (η = 0. 06), CoV (η = 0. 15), HRT SD (η = 0. 09), HRT SD/HRT (η = 0. 04), VAR (η = 0. 06), and VAR/HRT (η = 0. 01), significantly differed across groups, with the Invalid group having higher IIV values than all other groups. Results for ISD, HRT SD, and VAR remained significant even after controlling for each individual’s average performance. Higher IIV was often significantly associated with failure on individual PVTs. No IIV metric showed acceptable classification accuracy for ADHD vs. PD vs. ND, whereas all IIV metrics, except VAR/HRT, showed degrees of success in distinguishing groups from invalid performance (ISD: PD vs Invalid AUC = 0. 71; CoV: ADHD/PD/ND vs. Invalid AUCs = 0. 74/0. 80/0. 81; HRT SD: PD/ND vs. Invalid AUCs = 0. 74/0. 83; HRT SD/HRT: ND vs Invalid AUC = 0. 76; VAR: PD/ND vs. Invalid AUCs = 0. 70/0. 81). Results validated IIV metrics as sensitive to cognitive dysfunction and confirmed their utility for clinically distinguishing invalid performance from genuine attentional and psychological impairments among adults undergoing ADHD evaluation.

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