From these data, the project has generated the Regional Organizations Similarity Index (ROSI), a dyadic measure of the similarity between any two ROs at various points during their existence. We outline the rationale for ROSI and detail its construction, and show that it captures previously unstudied patterns of variation in the RO universe across time and space. In addition to generalizations about the case universe, ROSI allows us to estimate which institutional designs constitute deviations and which tend to follow established models. We demonstrate the validity of ROSI with the help of brief case studies exploring which institutional design features led to the identified scores.