The United States Census of Religious Bodies is, as the name suggests, a census of religious organizations, not a census of individuals (the U.S. Census collected data on religious organizations through the 1936 census). This census provides measures of the number of members in various denominations, by geographic unit. This is the third of four complete surveys on the subject of religious membership undertaken by the U.S. Bureau of the Census (preceded by the 1906 and 1916 censuses and followed by the 1936 censuses). The data are organized by county (counties are the cases).
The Bureau of the Census contacted the leaders of each identifiable denomination in the United States. The denominations supplied lists of churches, which were used to create contacts with local church leaders. Church leaders provided the membership statistics in this data (they also supplied other measures which are available in the paper version of the census). Churches that did not respond were sent several follow-up surveys and finally, if they still did not respond, were visited by a census worker.
Every identifiable denomination, based on lists of churches and religious organizations from yearbooks, denominations, and other sources, were contacted. Cults with no identifiable membership and interdenominational organizations were not included in the population of interest.
Department of Commerce and Labor, Bureau of the Census. Census of Religious Bodies, 1926, Part I: Summary and Detailed Tables. Washington, D.C.: Government Printing Office, 1930.