Although the number of intergovernmental organizations (IGOs) grew dramatically during the late 20th century, they have been part of the world scene for much longer. This data set tracks the status and membership of such organizations from 1815-2000. The IGO data sets contain information about intergovernmental organizations (that is, international organizations that have at least 3 nation-states as their members) from 1815-2000. The IGO data are collected at 5-year intervals from 1815-1965, and annually thereafter. The data are presented in three forms with distinct units of analysis. Form 1 is organized by IGO, listing all IGOs with the IGO-year as the unit of analysis, and identifying all state members of the IGO in that year. Form 2 is organized by state, listing all states with the country-year as the unit of analysis, and identifying all IGO memberships of the state in that year. Form 3 combines individual country memberships into joint dyadic memberships in each IGO, marking joint membership in each IGO for each pair of states.

The expanded version 2 updates the original Wallace and Singer (1970) data set to provide membership information from 1964-2000, along with recoding some earlier IGOs.

In any papers or publications that utilize this data set, users are asked to give the version number and cite the documentation for the data set, as follows (this citation will be replaced by the article of record for the data set when information is available):
Bibliography Entry: Jon Pevehouse, Timothy Nordstrom, and Kevin Warnke, "Intergovernmental Organizations, 1815-2000: A New Correlates of War Data Set."

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