The demands for migration data arising from the 2030 Agenda for Sustainable Development have prompted the international statistical community to review the use of traditional sources for migration data, such as population and housing censuses, household surveys and administrative records. There is also increased interest in looking for alternative sources to enhance the collection and analysis of migration data. The better use and understanding of existing data sources are essential to improve migration management and policy. Information about migration comes from a variety of data sources that have strengths and limitations and can be used to produce different migration statistics.

Diasporas, sometimes referred to as expatriates or transnational communities, play an important role in leveraging migration’s benefits for development. Measuring issues relating to diaspora groups is challenging, as there is no agreed-upon definition of "diasporas".  Data on migrant stocks can act as a proxy for diaspora populations, and remittance data are also closely linked to the study of diasporas. Since the 1990s, many states have established wide-ranging programmes aimed at promoting relations with diasporas. However, differences in definitions and a lack of robust monitoring and evaluation mean that comparative analysis of these policies is difficult.

 Reliable statistics on stocks or flows of irregular migrants, the well-being of migrants in irregular situations, or the extent to which they have access to services such as health and education, are generally not available. Irregularity does not refer to the individuals but to their migratory status at a certain point in time. Changes in national laws and policies can turn regular migration into irregular migration, and vice-versa. The status of migrants can change during their journey and stay in the country of transit/destination, which makes it difficult to have a comprehensive picture of irregular migration and the profiles of irregular migrants. Terms such as “irregular”, “undocumented”, and “unauthorized” are used interchangeably in this section.   

Migration forecasting is intended to provide informed guesses about future migration flows and trends. This is crucial information for policymakers to anticipate future challenges and adjust policies, design programmes and allocate resources. The term “forecast” is often used synonymously with “prediction” or “projection”, despite slight differences in meaning. Migration forecasts can be based on various and differing assumptions and results are prone to high uncertainty and errors because it is not possible to accurately predict the future.