Background: Patient-reported outcome measures (PROMs) are increasingly used in health care to evaluate service quality and client progress. Response scales are a critical component of PROM content validity and must be designed to be relevant and comprehensible by users. Methods: In collaboration with eight youth co-researchers with intellectual/developmental disabilities ages 14-21, we used an iterative, three-stage approach to develop and select a response scale for the PEDI-PRO. Stages: 1) inclusive development of response scale options; 2) Collecting data about response scale options during focus groups with youth with intellectual/ developmental disabilities (n = 62); and 3) Analysing data to refine response options. Results: Through two cycles of the three-stage process, the inclusive research approach led to the development of a content valid response scale that describes functional performance of everyday activities ("very easy," "a little easy," "a little hard"). Conclusion: An inclusive research approach can support the development of content valid PROM scales. We identified four broad strategies that supported youth co-researchers to engage in this response scale development process: universal design for learning, use of lived experiences, breaking down tasks, and peer support. Researchers may adopt and/or adapt the accessible inclusive research approaches described in this manuscript for measurement development and other research projects. [Co-written with the PEDI-PRO Youth Team.]