An expert on Islam, authors of a highly respected religion encyclopedia and analysts from the University of Chicago and the City University of New York all studied the same, straightforward question: How many Muslims are in the United States?
They couldn't agree on the answer.
Each issued a report this year on their findings, with vastly different conclusions -- ranging from estimates of just 1.8 million to 6 million.
Yet in the business of counting believers, for Islam or any faith, such discrepancies are not unusual. A mix of honest mistakes, bad data and wishful thinking make separating inflated numbers from real ones a difficult task.
''There are many pitfalls,'' said Todd Johnson, co-author of the World Christian Encyclopedia, among the more trusted sources of religion statistics. ''It depends on the methods you use.''
Since the U.S. census does not consider religion, researchers must use other means to determine the spiritual paths Americans follow.
High or at least growing numbers can help boost a denomination's national standing, and religious groups usually calculate their own membership figures -- using data that analysts say ranges from credible to insupportable.
The more established groups, such as the Presbyterian Church (U.S.A.) and the Church of Jesus Christ of Latter-day Saints, have their own respected research departments that regularly count membership.
The Roman Catholic Church, 62 million strong and the largest U.S. denomination, compiles an annual church directory and conducts other surveys on parishioners.
But for some smaller, less-organized faiths, the statistics can be inconsistent, derived in some cases from local clergy who have failed to purge church rolls of those who have left or died.
''For local pastors who give the information, you don't want to make yourself look any smaller than you are, so there's always a natural tendency to inflate,'' said Roger Finke, director of the American Religion Data Archive. Still, Finke believes the distortions are not dramatic.
Even the bigger denominations must critically review data each church provides, according to Tom Smith, of the National Opinion Research Center at the University of Chicago.
For example, pastors in the 15.9 million-member Southern Baptist Convention, the country's largest Protestant denomination, keep lists of what they call ''nonresident members,'' former worshippers the church cannot locate, sometimes for years.
Each church is autonomous, so researchers cannot demand that clergy clear their lists after a certain period, said Cliff Tharp, who helps compile the Southern Baptists' annual census. About 4.5 million ''nonresidents'' are included in the Southern Baptist national membership figure, he said.
Jack Marcum, a researcher for 13 years for the Presbyterian Church (U.S.A.), has had the tough job of reporting a decline every year in the mainline Protestant group, which now counts 2.5 million members.
Still, he says he feels no pressure from administrators to deliver more encouraging estimates. ''We take a hardline position that the numbers need to be accurate,'' he said.
Ministers at the denomination's 11,178 churches are required to provide statistics annually ranging from average worship attendance to church budgets.
Church leaders sometimes dissect the compiled figures looking for any positive trend, even when none exists, Marcum said.
''There's a lot of denial going on,'' he said.
The very definition of membership creates problems for researchers.
Roman Catholics include any baptized person who has not formally renounced his faith, according to Mary Gautier, a researcher with the Center for Applied Research in the Apostolate at Georgetown University.
''You don't have to be a good Catholic and you don't have to be a practicing Catholic,'' Gautier said.
Catholics are usually baptized as infants, so children are counted.
However, many Protestant denominations, such as the United Methodists, count someone as a full member only after confirmation, which usually occurs in early adolescence.
The analysis becomes even trickier when the topic is religious belief instead of affiliation. Often, people consider themselves part of a faith without ever joining a house of worship.
Professor Ihsan Bagby of Shaw University in Raleigh, N.C., was trying to address this issue when he calculated the highest estimate of the four reports on U.S. Muslims this year.
He concluded that about 2 million Muslims were active in mosques, then multiplied that number by three, since it is believed that few Muslims affiliate with Islamic centers. However, at the University of Chicago, Smith said the 6 million figure was ''untenable,'' and estimated the number was 2.8 million at most.
Census statistics about ethnicity and immigration can help researchers hone estimates. But some analysts make the mistake of assuming a person adheres to the predominant religion of his or her homeland, when often that is not the case, Johnson said.
For example, some 77 percent of Arabs in the United States are Christian, according to a survey last year by the Arab American Institute.
Often, Protestants consider themselves members of one denomination, but worship at the church of another, and can get counted as adherents of more than one group, Johnson said.
Many surveys also fail to ask critical follow-up questions, Smith said. If people identify themselves as Lutheran, researchers sometimes fail to ask if they are members of the Lutheran Church-Missouri Synod or the Evangelical Lutheran Church in America, Smith said.
Intermarriage poses another obstacle. Analysts sometimes wrongly assume that family members follow the same religion as the head of the household, skewing the count, according to Egon Mayer, a sociology professor at the Graduate Center of the City University of New York.
With all the pitfalls, how can any numbers be trusted?
Independent analysts say they check their information against multiple sources, from data on language and ethnicity to the numbers denominations tally themselves.
If several studies arrive at similar conclusions, they see a confirmation that the membership numbers are reliable, especially for the better-established, larger denominations.
''There are all these independent scholars out there who are trying different methods that are coming up with the same ballpark figures,'' Gautier said. ''So if you don't try to count to the last person, but instead you look at it as a sort of proportional thing, I feel pretty confident they're right.''
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