This analysis by John Green, of the University of Akron, and Beliefnet is based on the Fifth National Survey of Religion and Politics, conducted June-August 2008 by the University of Akron. The survey produced a national random sample of 4,017 adult Americans (with a margin of error plus or minus 1.5 percent).
The 12 tribes were defined in three steps. First, the survey respondents were sorted into the major religious traditions in the United States —Evangelical, Mainline, Latino and Black Protestants, Non-Latino and Latino Catholics, Jews, Unaffiliated and two composite categories of small groups, Other Christians and World Religions.
Second, an index of traditional religious beliefs—belief in God, life after death, view of scripture, evolution and the Devil; worship attendance, frequency of prayer, scripture reading and small group participation outside of worship; level of financial contribution to a congregation--was used to subdivide category the affiliation categories into those with high, medium, and low levels of religious traditionalism.
Finally, groups with similar religious backgrounds and levels of traditionalism were grouped into the twelve tribes. Sometimes a single category that was distinctive became its own Tribe. For example, the "religious right" are Evangelical Protestants with a high level traditional beliefs and practices, and "Convertible Catholics" were non-Latino Catholics with medium level of traditional beliefs and practices. But sometimes a variety of groups were put in the same tribe. For example, the "religious left" is made up of Evangelical Protestants, Mainline Protestants, and Catholics with low levels of traditional beliefs and practices.