http://www.wheaton.edu/isae/defining_evangelicalism.htmlHow Many Evangelicals Are There?
One of the most difficult things to establish about evangelicals is a precise estimate of just how many of them there are in the United States. With so many different evangelical denominations, evangelical constituencies of varying sizes within other historically evangelical "mainline" and even non-evangelical denominations, and thousands upon thousands of independent churches there is no single entity that can possibly serve as a representative gatekeeper for the nation's evangelicals. For that reason, the best guide to an evangelical head count is the cumulative result of various scientific surveys. But, even this is fraught with problems. As the discussion about the intricacies of definition above would indicate, the framing of the definition or wording of survey questions are important variables that can produce varying results. Estimates of the number of evangelicals in the United States, therefore, are just that: estimates.
Since 1976 the Gallup organization has been asking roughly 1,000 adults the question "Would you describe yourself as a 'born-again' or evangelical Christian?" In that first survey 34% of the people being surveyed responded "yes." Over the years, the number has fluctuated dramatically, reaching a low of 33% in 1987 and 1988 during the televangelist scandals, and a high of 47% in 1998.
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In its most recent sampling in 2001 approximately 40% of survey participants described themselves as evangelicals, compared to 45% the previous year. Over the years the Gallup numbers have averaged just under 39% of the population as accepting identification as born-again/evangelical.
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http://www.wheaton.edu/isae/defining_evangelicalism.html