Patterns of (dis)similarity in the design of regional organizations: The Regional Organizations Similarity Index (ROSI)

How similar are the institutional designs of regional organizations (ROs)? Is there a trend toward particular designs such as the European Union's, or is there greater institutional variety as more regions have created an increasing number of ROs? Which designs have spread through the system, and which remain idiosyncratic? To answer these questions, the Comparative Regional Organizations Project has assembled the most detailed dataset on ROs to date, with more than 80 organizations and their 276 founding and amending treaties being coded on more than 300 institutional design features. 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.