With a focus on the early stages of developing new assessment tools, we present an example of how researchers can apply qualitative data to the development of conceptual domains and specific items representing those domains for quantitative instruments. Specifically, our previous research examining mothers' perceptions and experiences of engaging in assisted exercise with their infants provided the foundation for the development of the Perceptions of Pediatric Activity Scale (PPAS). We describe the process we used to develop the PPAS as an exemplar for the process of incorporating qualitative data in instrument development. In addition, we address instrument development for diverse populations, and we provide examples illustrating how we extracted concepts using a concept-indicator model to construct the items in the PPAS. We conclude by noting that we are currently in the process of pilot-testing the PPAS to evaluate its utility and reliability.