Keeping track of people legally entering and leaving the United States is a formidable task. Nevertheless, the Department of Homeland Security (DHS) has released figures on people who overstay their visas and other legal forms of admissions. Total overstays for 2016 stood at close to 740,000 people, of which up to 630,000 were suspected to still be in the country.

You will find more statistics at Statista

As Statista's Dyfed Loesche notes, Canadians and Mexicans are the biggest groups of people with non-immigrant admissions to the United States that overstayed their lawfully authorized time period. However, the DHS only counts in arrivals and departures by sea and air as stated in its report.

Unlike all other countries, the overwhelming majority of visitors from Canada or Mexico enter the United States by land. "The collection of departure information in the land environment is more difficult than in the air and sea", the DHS writes. While many Canadians or Mexicans could fly in ore arrive by boat they might leave the U.S. across the land border.

So, there's always a degree of uncertainty in the data.

While Canada and Mexico are the United States' direct neighbors the figures for the rest of the countries shown in the below above probably are more accurate. This overview includes countries that are taking part in the so-called visa waiver program (VWP) and those who don’t. It only shows data for leisure and business visas, not for students.

The DHS admits that there is a level of uncertainty in how accurate these numbers are and calls them a snapshot. For the air and sea arrivals and departures the department relies on data that commercial and private carriers need to provide.

Also, the figures include suspected in-country and out-of-country overstays. This means that some of the people who initially overstayed might have already left.

All of which raises the question - does America need a northern border wall also?