The headline “710,000 migrants entered EU in first nine months of 2015” blared from a press release that year by Frontex, the European Border and Coast Guard Agency in Warsaw. Not so, said social scientist Nando Sigona, an expert on refugees and migration at the University of Birmingham, UK. Frontex, he pointed out, had been counting the same people two or three times or more — for example, a person who was recorded on arrival in Greece and left the EU by going to Albania was again counted on re-entering the bloc by a different route. Frontex has since made this caveat clear in its releases of cross-border data. But it is often the headline numbers that are retained by the media, and by the many populists and politicians who abuse data on refugees and migrants for political ends. We simply do not know the true figure.As I read that, I thought about a paper that my colleagues Justin Stoler and Piotr Jankowska and I published a few years on "Who's Crossing the Border" (referring to the U.S.-Mexico border). The U.S. Border Patrol gave us access to a database that included fingerprint IDs so that we could figure out the unduplicated counts of people apprehended at or near the border in the fiscal years 1999-2006. We found a consistent pattern of about 60% of total apprehensions representing the actual number of people. In other words, you need to downsize the reported numbers of people being apprehended in order to arrive at an actual picture of flows across the border. I suspect that this is true everywhere in the world.
Migrant stock is a little easier to estimate accurately if you have good census or survey data, but not all countries do, so there can be a lot of guesswork involved there, as well.