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Outcomes and outcome measurement instruments1255expert groups convened by the NIH,38,39 the ERICA PROMs Repository (Endo-ERN), and the establishment and validation of the National Institutes of Health Toolbox Cognitive Battery (NIH-TCB) for individuals with ID.27,41Type of reporterAlthough instruments are generally developed as one specific type of COA, similar instruments were completed by different types of reporters (e.g., a ClinRO instrument used as ObsRO by parents). Furthermore, proxy-reports were substantially more used (39%) than self-(assisted) reports (23%). Although the use of proxy-reports is not surprising in populations with ID, the validity of proxy reflections of unobservable internal states (e.g., anxiety or depression) is limited, as the personal perspective can only truly be understood by the individual’s self-report.42 Proxy-raters often assess (HR-)QoL worse compared to individuals themselves, indicating bias.43–49 PROs may thus be difficult to measure by proxy-reports,50 although still providing valuable information.51 It has been suggested that adolescents with ID can reliably report on their mental health status, with instruments appropriate to their age, cognitive and visuospatial functioning.52,53 However, a recent study illustrated that there is currently no self-report instrument available that is recommended for assessing (HR-)QoL and subjective wellbeing of adolescents with ID, based on psychometric evidence.42 Novel methods are upcoming such as experience sampling methods with the use of apps and ID-friendly instruments.54The emergence of condition-specific and personalized outcome measurement instrumentsIn an attempt to target more specific phenotypes without the need for multiple tools to measure the impact of the disease,55,56 condition-specific (n=18) and personalized (n=6) outcome measurement instruments, and tools particularly designed for a specific trial (n=16) were used. Conditionspecific PROMs have also been developed due to unavailability of proxyversions for adults with ID and criticism on appropriateness of the existing instrument’s content and measurement properties for the target population.55,57 Such instruments might contain more relevant items to complete, increasing acceptability among affected individuals. However, results might be difficult to generalize or interpret. Furthermore, it is not Annelieke Muller sHL.indd 125 14-11-2023 09:07