Analysis of the rarity of differences in FSIQ in the Italian sample of the WISC-V

The objective of this study is to investigate the unitarity of the Full Scale IQ (FSIQ) and three composites (General Ability Index - GAI, Cognitive Proficiency Index - CPI, Nonverbal Index - NVI) of the Italian adaptation of the WISC-V, aiming to determine their specific rarity thresholds. The importance of this aim is to determine if there is the possibility of using the FSIQ (or GAI, CPI, NVI) as a unique factor deviation quotient or not, by an accurate representation of the ability it is intended to assess. The distributions of the differences between maximum and minimum value (Max-Min discrepancies) were calculated using the Italian WISC-V standardization sample. The base rates of these discrepancies were analyzed, and statistical calculations of thresholds for large and rare differences were performed. The results confirm the validity of this statistical approach in determining the thresholds for the FSIQ and the indices of the WISC-V that corresponds to rare and unusual discrepancies. The obtained results combine the psychometric approach developed in previous versions of the Wechsler scales with the effective findings in the population as reflected by base rates (Flanagan & Kaufman, 2004, 2009; Orsini, Pezzuti & Hulbert, 2015). However, the FSIQ should not be classified as “uninterpretable” under any circumstances. Doing so would ignore its inherent predictive value, which remains intact regardless of score variability (Daniel, 2007).