The United Nations Population Division (UNPD) International Migrant Stock data provide an overview of the global stock of international migrants, including refugees, and allow for the comparison of countries and world regions.
The UNPD International Migrant Stock data provide the stock of international migrants in each country across time using intervals of five years. The main sources of information are the population censuses of individual countries (about 70% of the cases). In some other instances, the data are from population registers (about 17% of the cases) and nationally representative surveys (about 13% of the cases). In most cases the definition of the stock of international migrants is the stock of foreign-born residents (close to 80% of the countries), but the stock of foreign-nationals is used for some countries (close to 20% of the countries). In order to obtain estimates across years, UNPD applies methods of interpolation and extrapolation. At the time of publication, 2010 figures were based on extrapolations or projections from previous years. The figures provided are mid-year estimates (as of 1 July of the years indicated).
In countries where international refugees typically integrate to the population (mostly developed countries), the majority of refugees are counted by the population census. However, in many places refugees reside in camps. The census may fail to count refugees in these cases. The estimates from the Office of the United Nations High Commissioner for Refugees (UNHCR) are the main source of information on the number of international refugees in these countries. In 2007, UNHCR included in the estimates people in refugee-like situations (even if not recognized as refugees). These individuals left their countries before 2007, but only appear in the statistics at that date.
The factors that can make international comparisons problematic include differences in data collection practices between countries, the changing nature of borders and political/economic unions, irregular migration and varying degrees of naturalization of the foreign-born.
Stock estimates include migrations from some considerable time ago and do not capture the current patterns of migration, but the cumulative effects over the years.
Many countries (especially developing countries) do not allow refugees to become “regular” migrants or acquire citizenship, and therefore the estimates do reflect the stock of refugees over time.
Other countries, however, allow refugees to change status, hence, the estimates are likely to underestimate the true stock of refugees.