The method of data collection and selection used for the Calendar is consistent with that of the past 30 years.

Since 1910 the UIA has collected information on international organizations and their international meetings; a first statistical review was published in its Revue international des congrès in 1910. Since 1960 the UIA has prepared statistics on the international meetings that have taken place worldwide in the preceding year. This work is based on routine research and surveys undertaken by the UIA for the compilation of the Yearbook of International Organizations (published since 1950) and the International Congress Calendar (published since 1960).