“…conceptelicitation interviews can evaluate differences in patient experience across disease subtype [5,11] To help ensure accuracy of core outcome measures, the perspective of all stakeholders should be considered insofar as possible, especially patients and carers, e.g. concept-elicitation interviews can evaluate differences in patient experience across disease subtype [5,11] Need for recognition and buy-in of core outcome measures developed for specific disease areas, and time commitment to gather and include stakeholder perspectives Small, heterogeneous populations With small population sizes, it can be difficult to recruit enough patients for trials or PROM development/validation [13,21,22] Consider collaborating with patient advocacy groups and/or clinical care networks to maximize recruitment [22] Feasible: Requires collaboration stakeholder willingness for planning and time commitment but has the potential to save a substantial amount of time later in the process Consider using specialized statistical software that can work with small sample sizes while maintaining adequate psychometric properties [23] Feasible: Using statistical software depends on resources and knowledge, but using available tools to overcome as many data collection challenges as possible does not require substantial time or structural changes Multi-site/international data collection to pool samples and gain larger sizes [12,13,[24][25][26][27][28]45] Feasible with additional challenges: Multi-site/ international data collection is a good way to overcome the small sample size issue but poses challenges with regard to obtaining cross-cultural validity, and may thus require more consolidated and adhered to guidance to produce data of sufficient quality [13,33] Use of children-specific PROMs [33] Feasible: Proxy measures can and have been used, but care must be taken to ensure they are capturing the perspective of the patient as best as possible Use of observer-reported outcomes [32] Help of an interviewer when inability to selfreport is due to physical impairments [22,24,30,32] Need for recognition of challenges in collecting PROM data from certain patient populations and ensuring alternatives are accepted…”