For the purposes of this report, sectarian differences among Muslims were simplified into two categories: Sunni and Shia. It should be noted, however, that both these groups contain self-identified Muslim communities that may be considered heterodox or nonmainstream by other Muslims.
Unlike estimates for Muslim populations overall, almost no censuses and relatively few surveys ask Muslims about their Sunni or Shia affiliation. Accordingly, Pew Forum researchers have relied on three primary sources to generate Sunni-Shia estimates:
- Analyses by more than 20 demographers and social scientists at universities and research centers around the world who are acting as consultants on this project;
- Ethnographic analyses published in the World Religion Database (WRD); and
- A review of other published or frequently used estimates.
For most countries with sizeable Muslim populations, one or more experts provided the Pew Forum with their best estimate of the Sunni-Shia breakdown based on their own review of the published sources and other expert analyses available to them.
Additionally, for all countries and territories in the study, Pew Forum researchers consulted the WRD estimates of the proportion of Muslims who are Sunni and Shia in each country. The WRD estimates are based in turn on the WRD’s ethnicity database of more than 4,300 ethno-linguistic groups. Readers should note, however, that these estimates are limited both by the initial assumptions made about the Sunni-Shia composition of each ethnicity and by the variability of ethnicity information available in each country.
As a result, the Sunni-Shia estimates presented in this report are based primarily on data gathered via ethnographic and anthropological studies, necessitated by the fact that many Muslims either cannot or will not identify themselves as Sunni or Shia. Therefore, Pew Forum staff are not able to estimate the possible margin of error associated with any one particular estimate. Taking into account the three different sources, this study provides a likely range of the proportion of Muslims worldwide that are Shia based on an analysis of each country. Some ranges are broader than others because the sources consulted provided different estimates or because the sources suggest a wider range due to the lack of more precise information for a particular country.
Finally, for nearly 200 countries, Pew Forum researchers also consulted estimates contained in the religious demography section of the annual International Religious Freedom reports published by the U.S. State Department as well as estimates published in the CIA World Factbook.