How can you reliably estimate the number of people residing in a country illegally?

Since the late 1990s, the Ministry of Security and Justice has been attempting to monitor the size of the illegal immigrant population. It asked Methodology & Statistics researchers to provide these estimates.

Police data

Estimates of this kind can be made with the help of what is known as the ‘capture-recapture method’: estimating the total population based on the number of illegal immigrants who are arrested or detained.  This led to a number of reports, which have been published on the website of the Ministry of Justice.  The most recent report dates from the start of 2015.

We use a similar method when estimating numbers for the following groups:

  • perpetrators and victims of domestic violence
  • homeless people