U.S. borders are complex. They feature varied terrain, where disparate threats are met with varying levels of federal surveillance and resource allocation. Customs and Border Protection (CBP), within the Department of Homeland Security (DHS), is tasked with securing America's borders and promoting legitimate trade and travel. Within CBP, the U.S. Border Patrol (Border Patrol) is charged with securing the border between ports of entry. A reoccurring challenge for DHS and the Border Patrol has been establishing what a secure border means. Their efforts to develop a measure for this have created a continual evolution of metrics. Metrics can describe important phenomena, such as an estimate of how many unauthorized migrants enter the United States every year and what proportion is apprehended by the Border Patrol. These data can help evaluate the effectiveness of border security, influence decisionmaking, and inform Congress and the public. CBP has stated that "securing our borders means first having the visibility to see what is happening on our border, and second, having the capacity to respond to what we see." Furthermore, "Border Patrol characterizes a secure border as one of low risk, where there is a high probability of detection coupled with a high probability of interdiction." Border security can be assessed at both the strategic and operational levels. At the strategic level, DHS can use performance metrics to provide understanding on whether efforts to secure the border are effective or efficient. These metrics also hold DHS and its components accountable and can affect its decisionmaking. At the operational level, Border Patrol uses the identification of risk and the estimation of its magnitude in its day-to-day operations to secure the border. Such assessment helps the Border Patrol align its resources against specific threats and can help describe how secure a border sector is. It is unclear whether metrics at these two levels interact with one another and if they do, how they influence or affect one another. Understanding the security risks at the border and developing methods to measure the performance of the Border Patrol in allaying these risks are fundamental to congressional oversight of the Border Patrol, CBP, and DHS more broadly. Some Members of Congress have asked for a clear measure of how many unauthorized migrants cross the border and/or of the overall state of border security. However, determining unauthorized border flows is difficult for many reasons, such as unauthorized migrants' desire to avoid detection. Still, metrics, at both the strategic and operational levels, ideally provide Congress with a sense of the current state of the border that helps inform public debate and possible legislation. For example, in past discussions around comprehensive immigration reform, some have said that legislation must include measures to increase border security, while others have urged that the border first be secured before other immigration reforms take place. This report reviews the use of metrics at the department (i.e., DHS) and agency level (i.e., CBP ). The report begins by describing the performance metrics at the strategic level that DHS is developing (the level of unauthorized entry of migrants) and has used (the recidivist rate and the effectiveness rate). Next, the report summarizes DHS's, and the former Immigration and Naturalization Service's (INS's), use of metrics since the early 2000s. The report also addresses the Border Patrol's metrics at the operational level. It concludes with a discussion of select issues Congress may consider when evaluating measures of border security. The Appendix includes information on additional data and methodologies that have been reviewed or recommended by scholars and think tanks. DHS's, and the former INS's, efforts in developing different performance metrics at the strategic level to depict the state of the border have resulted in an array of measures that focus either on border security as a whole or on a component of it ( Figure 1 ). Over time, performance metrics have come and gone, offering little continuity and opportunity for historical assessment. In recent years, DHS has been developing a new measure to estimate unauthorized entry of migrants into the United States. While DHS has shown intent to continually improve its measurement of border security, this new metric is subject to influence by external factors that are outside of the control of DHS and may not directly measure border security. DHS currently reports the recidivism rate and the rate of interdiction effectiveness. These measures, though subject to limitations, are meant to estimate the Border Patrol's effectiveness in apprehending migrants and deterring migrants from re-entering the United States. DHS's continual re-evaluation of its performance metrics has created a history of past metrics that have either attempted to comprehensively measure border security, such as optimal deterrence and operational control, or have reported only single metrics, such as apprehensions. DHS has been working to create a new generation of performance metrics in an effort to better measure border security at the strategic level. For instance, in 2011 DHS announced that CBP was developing a new "border conditions index" (BCI), which would have included measures of estimated unauthorized flows between ports of entry, wait times to cross the border at ports of entry, the efficiency of legal flows at ports of entry, public safety, and quality of life in border regions. In 2013, DHS officials determined that the BCI would not satisfy the demands for a single comprehensive measure of border security and discontinued its development. Abandoning efforts to establish a single, comprehensive measure of border security, in FY2015 DHS began to develop a means of estimating another component of border security: the level of unauthorized entry into the United States. The measure would only cover migration, not drug smuggling or other security concerns. It appears that DHS opted to fashion what may be a baseline figure upon which it can build broader, more nuanced estimates of performance. Through the use of this metric, DHS hopes to inform its strategic efforts and decisionmaking, while also informing the public and Congress. According to DHS officials, the agency has begun efforts to estimate the level of unauthorized entry of migrants into the United States. DHS commissioned the Institute for Defense Analyses (IDA) to produce a baseline estimate for this new metric through the use of the repeated trials method (RTM), also known as the capture-recapture method. The basic model calculates the probability of apprehension by examining a group of migrants who have been apprehended and removed previously, with the assumption that the migrant will continue to attempt crossing the border. The probability of apprehension for a given trip is calculated as: The probability of apprehension can then be used to estimate the level of unauthorized entry by: An advantage of RTM is that it relies on observable administrative enforcement data—apprehensions and recidivists—to calculate key border security metrics: apprehension rates and unauthorized flows. The model has limitations grounded in certain assumptions, such as that the pool of migrants is fixed, the probability of being apprehended is the same for each attempt, and those who have been arrested have not succeeded in entering the United States between arrests. The basic model also does not account for migrants who are deterred from attempting to cross the border again because it uses the basic assumption that migrants will continue to attempt crossing the border until they are successful. Furthermore, given that this metric measures migrant volume and is influenced by the push and pull factors of migration that are outside the control of DHS's components (e.g., economic, social, and political factors in source countries and countries of destination), it does not speak directly to the effectiveness of border security efforts. DHS is working to factor deterrence into RTM in order to control for migrants who will be deterred from attempting to re-cross the border. DHS is using a survey that measures the characteristics, volumes, tendencies, and effects of migrant flows between the United States and Mexico. The survey, Encuestas sobre Migración en la Frontera Norte de Mexico (EMIF Norte), is conducted by the Colegio de la Frontera Norte (COLEF) and other agencies of the Mexican government, who sample potential migrants at busy departure points in Northern Mexico and migrants removed from the United States. With the incorporation of deterrence, the probability of deterrence is then calculated as: EMIF Norte provides information on other migrant populations, outside of Mexicans. RTM models are most useful when sensitive to how migrants from other source countries could affect results. In addition to RTM, DHS is developing two other methods to validate its estimates. One method is based on the Encuesta Nacional de Ocupación y Empleo (ENOE), a representative Mexican household survey. The second method is based on an econometric analysis of apprehension records used to validate deterrence. These new measurements are to be included in DHS's State of the Border report that is to be produced alongside its 4 th Quarter BorderStat Report. DHS also plans to create "BorderStat," modeled on law enforcement's CompStat, which would provide current and historical trends for performance metrics along with statistics specific to subpopulations among the unauthorized population who enter the United States. In addition to estimating the level of unauthorized entry of migrants into the United States, DHS has also begun to turn its attention to evaluating the performance of the technology used to secure the border. A 2014 Government Accountability Office (GAO) report found that CBP had identified how technologies contribute to its mission but that it had not yet created performance metrics to evaluate them. DHS databases allow for CBP to collect data on "asset assists," which are "instances in which a technology, such as a camera, or other asset such as a canine team, contributed to an apprehension or seizure." Currently, Border Patrol agents are not required to record these data and therefore the data on asset assists are incomplete. GAO stated that "requiring the reporting and tracking of asset assist data could help CBP determine the extent to which its surveillance technologies are contributing to CBP's border security efforts." According to DHS officials, the agency's FY2016 border research agenda will include an analysis on the effectiveness of border investments on outcomes.