"The relationship between border security and unauthorized migration is a key issue for many people interested in immigration reform. Two questions loom large in this discussion: how many unauthorized migrants enter the United States? Of those attempting entry, how many does U.S. Customs and Border Protection (CBP) apprehend? These questions sound simple, but they are difficult to answer for the obvious reason that unauthorized aliens seek to avoid detection. This missing information means analysts do not know the precise scope of the illegal migration problem, nor can they calculate CBP’s enforcement success rate. These challenges are well known. Several members of this Committee have called on the Department of Homeland Security (DHS) to develop clear, measurable, outcomes-based metrics to evaluate progress with respect to immigration control. Unfortunately, the illicit nature of unauthorized migration along with the complexity of DHS’s border security mission and the size and diversity of U.S. borders mean that no single, quantitative, off-the-shelf indicator accurately and reliably provides a metric or “score” for border enforcement. Instead, we assess border security by estimating unauthorized flows and apprehension rates, and there likely will always be some disagreement about these estimates. Nonetheless, researchers have done substantial work on how to make such estimates." - Congressional Research Service, House Testimony, Feb. 26, 2013.