Source: The New York Times
On September 18, 2002, The New York Times reported on "the study, 'Religious Congregations and Membership: 2000'... Because the Census Bureau does not ask about religion, some scholars regard this study, first done in 1971, as the most comprehensive assessment available of the changes in American religious affiliation. The study is based on self-reporting by religious groups, a method that the study's authors acknowledge is imprecise because religious groups can inflate their numbers. The study was conducted by Glenmary Research Center... The 2000 study is the first to include information on religious groups other than Christians and Jews... The numbers of Muslims and Jews reported in the survey could be misleading. The estimate of Jews was 6.1 million, but the count included Jews who are unaffiliated with synagogues - the only group in the survey to use identity and not membership as its criteria in the count... The estimate of Muslims was 1.5 million, derived by counting the members reported by a third of the nation's 1,200 mosques, which often do not maintain membership rolls. Because some Muslims are new immigrants and others are recent converts, reliable estimates are difficult... The study's number is far lower than the seven million claimed by most American Muslim groups."