The severity of need index was developed based on feedback and input from academic experts in the fields of HIV/AIDS surveillance, medical treatment, and service delivery; Ryan White Program administrators from parts A and B, and federal Ryan White Program staff. To be considered for inclusion in the index, a measure had to be quantitative, collected in the same way across all program areas, updated regularly, understandable by the general public, and reproducible given the data sources and methodology used to construct it. Although the measures themselves were scientific, their combination and weighting involved policy decisions. These decisions can be modified over time to meet the changing needs of the program. Future refinements to the SON Index will be made to incorporate additional scientific evidence as it becomes available.