A new survey, and a study of other polls and surveys, indicates that the Muslim population of the United States may be smaller than has previously been widely estimated.
Scholarly estimates, much cited in recent weeks, have put the Muslim population in the United States at as high as six million.
But a survey of religious affiliation among American adults, released yesterday by the Graduate Center of the City University of New York, estimated that there were 1.1 million Muslim adults living in the United States (more than double the 500,000 Muslims found in a similar, sweeping survey done by the Graduate Center in 1990).
In an interview, Egon Mayer, a sociologist at the Graduate Center and Brooklyn College who directed the study with the sociologist Barry Kosmin, estimated the total American Muslim population, based on the findings, at 1.8 million adults and children. Of the 1.1 million adult Muslims, 17 percent are converts, the survey estimates.
The survey, the American Religious Identification Survey 2001, was based on random interviews from February through June with more than 50,000 people, said Ariela Keysar, a demographer at the Graduate Center who worked on the survey. Among its main findings are that 52 percent of adults identify themselves as Protestants, 25 percent as Roman Catholics, 1.3 percent as religious Jews. An additional 14 percent said they had no religion.
Another report came up with slightly different numbers.
The report, commissioned by the American Jewish Committee and confined to the Muslim population in the United States, estimates that there are at most 2.8 million Muslims making up no more than 1 percent of the American population. It was written by Tom W. Smith, director of the General Social Survey at the National Opinion Research Center at the University of Chicago, who said that most estimates of the nation's Muslim population were not based on scientific methods.
Though the committee's report was compiled from independent surveys, it was unclear whether, given its sponsor, it would be accepted by some Muslim groups.
David A. Harris, the Jewish committee's executive director, said it sought the report after seeing "wildly divergent" figures being reported on the American Muslim population after the Sept. 11 terrorist attacks.
The report, titled "Estimating the Muslim Population in the United States," was the fifth in a series on religious issues the committee has asked the research center to do since 1991.
In the report, Dr. Smith wrote that the "best, adjusted, survey-based estimates" — the General Social Survey, Gallup polls and other national surveys — indicated that 1.4 million adults identify themselves as Muslims. On that basis, he estimated a total Muslim population at 1.9 million adults and children. But Dr. Smith also wrote that other surveys indicated that there might be as many as 2.1 million Muslim adults in the United States, or 2.8 million adults and children, a figure equivalent to 1 percent of the total American population.
Two major American Muslim organizations offered somewhat differing responses to the reports when informed of their findings.
Ibrahim Hooper, national director of the Council on American-Islamic Relations, said that Muslim population estimates varied because substantial numbers of Muslims "don't get caught up in a statistical net" when such surveys are conducted. "Recent immigrants are going to be far less likely to be accessible in this way," Mr. Hooper said, referring to telephone surveys.
In April, the council published a study reporting the number of mosques rose by about 25 percent, to more than 1,200, from 1994 to 2000. Based on reports of attendance at some mosques, some researchers estimated the number of American Muslims at 6 million to 7 million.
But Salam Al-Marayati, director of the Muslim Public Affairs Council, said the actual number of Muslims in the United States was far less important than the role that American Muslims played in society.
"What's important is the level of understanding, the ability to include the Muslim voice within the American mosaic, and clarifying the direction that the American Muslim community will pursue with both domestic and international policy," Mr. Al- Marayati said.