1. There is a fundamental asymmetry in international migrations. The right to leave the country in which one resides is, nowadays, established as a fundamental individual liberty. On the contrary, the entrance on a country, other than that of nationality, still remains to be dependent on the sovereign will of national states. Consequently, in norm, there are no administrative records of exits (emigration), but only of entries (immigration). Estimate and characterize the emigration of a country, therefore requires the compilation of records based on the entrance and sojourn of the emigrants on the destination country. The data that the Observatory of Emigration collects, disseminates and analyses, is therefore, the data obtained from the institutions responsible for the immigration statistics on the destination countries of the Portuguese emigration. That data is compiled, by country of destination (on the section Destination countries), and by indicator (in this section “Data”, subsections Annual series and Census).

2. It is based on the data on Portuguese entries in the countries of destination, compiled by the Observatory, that the annual estimates on total Portuguese emigration are drawn up, generally higher than the estimates that are of responsibility of the Instituto Nacional de Estatística (Statistics Portugal)(INE) (both available in this "Data" section, Global estimates). The main source of information used by INE to estimate international migrations to and from Portugal, is the Inquérito ao Trabalho (Labour Force Survey), conducted by sampling. In the case of immigration, the results thus obtained are subsequently corrected with the administrative records of the entries (data from Serviço de Estrangeiros e Fronteiras). For emigration, not only is there, as mentioned, national administrative registers that allow a similar adjustment, as the inquiry procedure is indirect, since those who have left can not, by definition, be questioned. The result of this asymmetry in statistics on exits and entries has resulted in an understatement of the numbers of permanent Portuguese emigration when compared to those calculated by the Observatory from the statistics of Portuguese arrivals in destination countries. These being the data published by Eurostat and used for the calculation of migratory balances (also available in this section "Data", subsection Global estimates), is probable that these balances are lower than those recorded in the national and European statistical system.

3. The problems of harmonization the data produced by such a wide range of organizations, the fragility of the statistical systems in some countries, as well as the international nature of the migratory movements at stake, are in the origin of a bigger intervention of the main international organizations in the production of databases and statistical indicators on emigration. Such data is now indispensable consultation for a better knowledge of the migratory phenomenon. Data and estimates on Portuguese emigration available in sources such as Eurostat, OECD, the World Bank and the United Nations, are also compiled in this website (section “Data”, subsection Global estimates).

4. In the international organisms, there is now a consensus on the use of the variable place of birth as an indicator of emigration: an emigrant is someone who has lived for more than a year in a country other than the one in which he/she was born. However, when it comes to statistical on the entry of immigrants into a country, the available data is often data on the entry of foreigners, since it is the control of entry of foreigners that is object of record. The Observatory uses, therefore, on this website and its publications, two indicators on emigration. When it comes to measuring and characterize the emigrant Portuguese population, the retained indicator is, whenever available, the place of birth – born in Portugal living in other countries. When it comes to measuring the movement of Portuguese entries in destination countries, the used indicator is normally, the nationality – Portuguese inflow in other countries. These and other methodological issues, as well as the precise identification of all the sources consulted, are dealt in this section “Data”, subsection Metadata.

5. In result of the activity either by the international organisms, either mainly, of the investigations projects on Portuguese emigration, there are now databases (microdata) that constitute a valuable resource for the production of new analysis in this domain. The links for those databases, or even, its availability in this website, on some cases free, on others conditioned, are available on this section “Data”, subsection Databases.