"Securing the southwest land border against illegal immigration, smuggling of drugs and other contraband, and terrorist activities is a key part of the Department of Homeland Security's (DHS) mission, and a top priority for the White House, Congress, and the American public. On January 25, 2017, President Trump signed an Executive Order on 'Border Security and Immigration Enforcement Improvements,' reflecting these concerns. In light of the effort and resources the Department has devoted to border security in recent decades, as well as the sustained public attention to the southwest border, Congress has directed the Department to provide more detailed reporting on southwest border security. The Consolidated Appropriations Act, 2017 directs the Department to publish 'metrics developed to measure the effectiveness of security between the ports of entry, including the methodology and data supporting the resulting measures.' While DHS employs a number of concrete metrics to track border security operations, it is difficult to precisely quantify illegal flows because illegal border crossers actively seek to evade detection, and some flows are undetected. As a result, any effort to quantify illegal flows or calculate an overall enforcement success rate must rely on one or more estimation techniques. Measurement is also difficult because of the diversity and complexity of the enforcement mission along the United States' 2,000-mile land border with Mexico."
U.S. Department of Homeland Security: https://www.dhs.gov/