This survey was conducted for ABC News, the BBC and NHK by D3 Systems of Vienna, Va., and KA Research Ltd. of Istanbul, Turkey. Interviews were conducted in person, in Arabic or Kurdish, among a random national sample of 2,212 Iraqis aged 18 and older from Aug. 17 to Aug. 24, 2007.
Four-hundred-fifty-seven sampling points were distributed proportionate to population size in each of Iraq's 18 provinces, then in 101 of 102 districts within the provinces (excluding Sinjar district in Ninevah province, which was not accessible because of security concerns at the time of the survey), then by simple random sampling among Iraq's nearly 11,000 villages or neighborhoods, with urban/rural stratification at each stage.
Maps or grids were used to select random starting points within each sampling point, with household selection by random route/random interval and within-household selection by the "next-birthday" method. An average of five interviews were conducted per sampling point. Sixteen of the 457 sampling points were inaccessible for security reasons and were substituted with randomly selected replacements.
Interviews were conducted by 117 trained Iraqi interviewers with 30 supervisors. Sixty-nine percent of the interviews were supervised or reviewed by supervisors -- 44 percent by direct observation, 16 percent by revisits and nine percent by phone. All questionnaires were subject to further quality-control checks.
In addition to the national sample, oversamples were drawn in Anbar province, Sadr City, Basra city and Kirkuk city to allow for more reliable analysis in those areas. Population data came from 2005 estimates by the Iraq Ministry of Planning. The sample was weighted by sex, age, education, urban/rural status and population of province.
The survey had a contact rate of 93 percent and a cooperation rate of 65 percent for a net response rate of 60 percent. Including an estimated design effect of 1.51, the results have a margin of sampling error of 2.5 percentage points at the 95 percent confidence level.
Given the attitudinal differences between Shiite and Sunni Arabs in Iraq, there's interest in the relative sizes of these two groups. We find no official Iraqi estimate of the country's Sunni vs. Shiite Arab populations and no authoritative source of empirical data on the subject.
The most commonly cited estimate is an unsourced reference in the CIA World Factbook; saying that 60 percent to 65 percent of Iraqis are Shiite Muslims, 15 percent to 20 percent Kurds and 3 percent non-Muslims. Though not explicitly stated, that leaves room for 12 percent to 22 percent Sunni Arabs.
This estimate may be derived from a 1988 book, "Iraq: a Country Study" produced by the Federal Research Division, Library of Congress. This book (pp. 80-81) characterizes data on ethnicity and religious doctrine in Iraq as "not absolutely reliable." It says, "Officially the government sets the number of Shias at 55 percent. In the 1980s knowledgeable observers began to question this figure, regarding it as low. ... A more reasonable estimate of their number would seem to be between 60 and 65 percent." It adds, "The Sunni Arabs ... constitute a decided minority of only about 13 percent."
These data also are unsourced. The 60 percent to 65 percent Shiite estimate matches that in the CIA World Factbook; the 13 percent Sunni Arab estimate compares to the World Factbook's unstated range of 12 to 22 percent.
Data from recent random-sample surveys, including the last two sponsored by ABC News with media partners, contrast with these unsourced estimates. This survey puts Iraq's population at 48 percent Shiite Arab, 33 percent Sunni Arab, 16 percent Kurdish and three percent other. The previous survey, in March, was almost identical: Forty-seven percent Shiite Arab, 35 percent Sunni Arab, 15 percent Kurdish and three percent other.
These two surveys together comprise more than 4,400 interviews from 915 sampling points, a large sample with an unusual level of geographical coverage.
Iraq surveys from other sources are difficult to compare; some ask religious doctrine different ways, often reporting significant numbers of Muslims of unspecified doctrine; some use different weights, including weighting to a predetermined assumption of distribution by religious doctrine. In the absence of other persuasive empirical data, this poll did not weight Iraqis' religious doctrine to any assumed target.