Examining the validity of the use of ratio IQs in psychological assessments.

Intelligence tests are amongst the most used psychological assessments, both in research and clinical settings. To avoid missing data points, for participants who cannot complete Intelligence tests normed for their age, ratio IQ scores (RIQ) are routinely computed and used as a proxy of IQ. Here, we use the case of autism to examine the validity of this widely used, yet never scientifically validated, practice. We examine the differences between standard full-scale IQ (FSIQ) and RIQ. Data was extracted from four databases in which age, FSIQ scores and subtests raw scores (from which RIQ scores could be calculated) were available for 16,751 autistic participants between 2 and 18 years old. The Intelligence tests included were the MSEL (N = 12,033), DAS-II early years (N = 1270), DAS-II school age (N = 2848), WISC-IV (N = 471) and WISC-V (N = 129). RIQs were computed for each participant as well as the discrepancy (DSC) between RIQ and FSIQ. We performed a multiple linear regression model to assess the effects of age and FSIQ on DSC for each IQ test. Participants at the extremes of the FSIQ distribution tended to have a greater DSC than participants with average FSIQ. Furthermore, age significantly predicted the DSC, with RIQ superior to FSIQ for younger participants while the opposite was found for older participants. Similar results were found in secondary analyses including typically developing children. These results question the validity of the RIQ as an alternative scoring method, especially for individuals at the extremes of the normal distribution, for whom RIQs are most often employed.