Countries estimate how many people move in and out using censuses, surveys, and border records. How accurate are these numbers, and can they account for illegal migration?

Debates about migration are often in the news. People quote numbers about how many people are entering and leaving different countries. Governments need to plan and manage public resources based on how their own populations are changing.

Informed discussions and effective policymaking rely on good migration data. But how much do we really know about migration, and where do estimates come from?

In this article, I look at how countries and international agencies define different forms of migration, how they estimate the number of people moving in and out of countries, and how accurate these estimates are.

Migrants without legal status make up a small portion of the overall immigrant population. Most high-income countries and some middle-income ones have a solid understanding of how many immigrants live there. Tracking the exact flows of people moving in and out is trickier, but governments can reliably monitor long-term trends to understand the bigger picture.

Who is considered an international migrant?

What about illegal migration?

The difference between immigration flows and stocks

How are immigration stocks measured?

How are immigration flows measured?

One example: the United Kingdom

...read more at ourworldindata.org