1990 data were collected by the Association of Statisticians of American Religious Bodies (ASARB) and include statistics for 132 religious groups, including number of churches and adherents. Martin B. Bradley, Norman M. Green, Jr., Dale E. Jones, Mac Lynn, and Lou McNeil supervised the collection. These data originally appeared in Churches and Church Membership in the United States, 1990, published by the Glenmary Research Center.

The Association of Statisticians of American Religious Bodies (ASARB) invited all Judeo-Christian religious bodies listed in the Yearbook of American and Canadian Churches to participate. Final totals include information from 133 Christian and other religious bodies. The 133 groups reported 255,173 congregations with 137,064,509 adherents, which is 55.1% of the total population of 248,709,766. Five groups (Black Baptists; Congregational Christian Churches [not part of any national CCC body]; Independent, Charismatic Churches; Independent, Non-Charismatic Churches; and Jewish adherents) are not denominations or fellowships, but account for 12.2 percent of the adherents in the United States. There are 21 non-participating church bodies that reported more than 100,000 members to the Yearbook of American and Canadian Churches. These groups reported a combined membership of 20.8 million in the Yearbook.

The RCMS collection reports a measure of members and adherents. Members include only those who are designated as full members by the congregation. Congregational "adherents" include all full members, their children, and others who regularly attend services or participate in the congregation. When religious groups reported only adult membership, the following formula was used to derive the number of adherents: The total county population was divided by the total county population less children 13 years and under (derived from census), and the resulting figure was multiplied by the confirmed members. Using adherents allows for more meaningful comparisons between groups that count children as members (e.g., Catholics) and those that don't (e.g. Baptists).

William Newman and Peter Halvorson provided county estimates of the Jewish population for this study, based upon the American Jewish Year Book. Generally, a county was included in the study only if it or its metropolitan area contained at least 100 adherents. Barry Kosmin provided a list of synagogues throughout the United States. These have been located by county and are included in the study. Together, the reports indicate nearly 6 million adherents in 3,975 synagogues. Due to the differing reporting procedures, some counties report adherents with no synagogues, and others report synagogues but no adherents.

Three predominantly black denominations, accounting for 1.2 million adherents, participated in the study. There is no way of telling how many blacks are adherents of the other denominations appearing in the study. Major efforts to enlist the participation of other black denominations were made. Generally, membership records are not kept nationally, at least not in a form conducive to participation in a study such as this.

Christian groups in America are often placed into four main traditions: Evangelical Protestant, Mainline Protestant, Historically Black Protestant and Roman Catholic. There is wide variation within each of these four traditions. The most difficult to classify are evangelical and mainline Protestants, with evangelical groups generally being more theologically and socially conservative than mainline groups. Several sources were used for categorizing the many Christian groups into religious traditions, beginning with an article by Brian Steensland and colleagues entitled "The Measure of American Religion: Toward Improving the State of the Art" published in Social Forces (2000, 79: 291-318). This article places most denominations into one of above traditions. For Christian denominations not included in Steensland and colleagues' scheme, two additional sources were consulted: J. Gordon Melton's Encyclopedia of American Religions (8th edition) and Frank S. Mead and Samuel S. Hill's Handbook of Denominations in the United States (10th edition). In addition to those in Steensland and colleagues' classification scheme, separate categories were created for Orthodox Christians, Jews, and other groups. The lack of data on historically African American denominations made inclusion of a Black Protestant category unnecessary. Using these sources, staff at the Association of Religion Data Archives (theARDA.com) placed each group into a Religious Tradition.

The 133 religious groups and their category placement are as follows: