Methodology for Muslim Population Estimates
The Pew Forum's Mapping the Global Muslim Population: A Report on the Size and Distribution of the World's Muslim Population seeks to provide the most up-to-date and comprehensive demographic estimate of the number of Muslims in the 232 countries and territories for which the United Nations Population Division provides general population estimates.1 In order to have statistics that are comparable across countries, wherever possible this study counts all groups and individuals who self-identify as Muslim. (The method for identifying Sunnis and Shias is different; see Methodology for Sunni-Shia Estimates for a complete explanation.)
The number of Muslims in each of the countries and territories is calculated by multiplying the United Nations' 2009 total population estimate for each country and territory by the single most recent and reliable demographic or social-scientific estimate of the percentage of Muslims in each country's population, based on the conservative assumption that Muslim populations are growing at the same rate as each country's general population. (A 2010 Pew Forum report will provide estimates of the differential growth rates of Muslim populations.)
Sources include national censuses, demographic and health surveys, and general population surveys and studies. The specific source used for each country is indicated in Data Sources by Country. Readers should note, however, that general population surveys generally have smaller sample sizes than demographic surveys and are not designed to measure the size of small minority populations. This may lead to undercounts of Muslims in countries where they represent a small minority of the population and overcounts where they represent the vast majority of the population. See below for more detail.
With all sources, results may have been affected by methodological decisions with respect to how the data were collected and managed. Social, cultural or political factors could also have affected how answers to census and survey questions are provided and recorded.
Discussion of Sources
For this study, Pew Forum researchers acquired and analyzed religious affiliation data from 81 censuses that were conducted since 1999, comparing more current sources of data with older census data on religious affiliation for an additional 103 countries as a cross check. Religious affiliation questions from national censuses are the best source for estimating the number of Muslims because they generally cover the entire population and are conducted on a fairly regular basis. The chief limitation in using census data is that fewer than half of recent country censuses included a religious affiliation question. In addition, these surveys are conducted only once every 10 years.
Where recent census data on religion are not available, religious affiliation questions from large-scale demographic surveys, such as Macro International's MEASURE Demographic and Health Surveys (DHS), are the second-best source because of their large sample sizes, sampling frame and representative results at the province level. Though less comprehensive than census data, demographic surveys complete sufficiently high numbers of household interviews to produce a generally accurate demographic profile of the country. For this report, DHS data were acquired and analyzed for more than 60 countries, or nearly two-thirds of the countries where census data are lacking or are older than 1999. For most of the DHS surveys, both women and men are interviewed and Macro International provides the data in separate male-female datasets. Pew Forum staff pooled the female and male datasets in consultation with sampling experts at Macro International so that the combined dataset retains nationally representative results. In countries where only females are interviewed, Pew Forum staff used those data to make the overall Muslim population estimate for the country.
General Population Surveys
Pew Forum researchers acquired and analyzed religious affiliation data from general population surveys for some 100 countries. In more than 20 of those countries, these surveys provide religious affiliation data where a recent census or demographic survey is lacking. Since general population surveys typically involve only 1,000 to 2,000 respondents, however, they provide less accurate numbers. This is especially true where the size of the Muslim population is quite small or Muslims live in concentrated locations that are not oversampled. As a result, data drawn primarily from these sources is marked with an asterisk in the tables throughout this report.
World Religion Database
Pew Forum researchers also used estimates from the World Religion Database, primarily for countries where census and survey estimates were out-of-date, unavailable or lacked sufficient coverage. Besides census and survey reports, WRD estimates also take into account other sources of information on religious affiliation, including anthropological and ethnographic studies as well as reputable statistical reports from religious groups themselves. The WRD is an outgrowth of the international religious demography project at Boston University's Institute on Culture, Religion and World Affairs.
A Note on Country and Territory Designation
The word "country" in this report refers to all countries recognized as such by the United Nations. The word "territory" in this report does not have a technical definition, but rather is a general term for distinct geographical entities not recognized as countries by the United Nations but that have separate population estimates reported by the United Nations. Territories in this report include such entities as Hong Kong and Macau (special administrative regions of China), Greenland (an autonomous constituent country within the Kingdom of Denmark) and the Commonwealth of Puerto Rico (an unincorporated territory of the United States).
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1 Population estimates for 2009 for Taiwan and Kosovo are from the Population Reference Bureau. Taiwan's population is deducted from the U.N.'s China estimate and Kosovo's from the U.N.'s Serbia estimate.