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When the Department of Homeland Security trudged up to Capitol Hill this year and asked lawmakers for another 365 days worth of funding, as every federal agency does each year, officials ticked off a list of accomplishments they presented as evidence of the agency’s ongoing success.

The department pointed to a marked decrease in the number of immigration apprehensions by the Border Patrol. Apprehensions, they said, are “a key indicator of illegal immigration,” according to its annual budget request totaling $39.5 billion for the 2013 fiscal year.

Such apprehensions are down more than 50 percent since 2008, and total just one-fifth of what they were in 2000, the request claims.

But a downward trend in total apprehensions may not prove the department’s border enforcement strategies are working, because they don’t reflect the number of people who elude capture or who are discouraged from attempting to cross again after being caught once.

That’s according to a new report from the National Academy of Sciences commissioned last year by the Homeland Security Department, which was looking for better ways to estimate the number of illegal crossings at the U.S.-Mexico border. The Border Patrol, said one of the report’s authors, can easily take credit either way.

“DHS might point to (apprehensions) as a measure of success whether they go up or down,” said Peter Brownell, an associate social scientist at the Santa Monica-based Rand Corporation who worked on the report. “If they go up, it means we’re doing a better job of catching people. If they go down, it means we’re doing a better job of deterring people. They’re not a very useful performance statistic unless we can parse out the changes in probability of apprehension from changes in flow.”

While the decline in apprehensions has coincided with ramped up efforts by the department to secure the border – from using pricey new surveillance systems to doubling the number of Border Patrol personnel – the two are not clearly linked to one another.

Furthermore, the Border Patrol isn’t the only agency responsible for enforcing immigration laws. There’s also Immigration and Customs Enforcement and the Office of Field Operations. All three are located inside the Department of Homeland Security. Data from each on immigration enforcement actions has not been combined and widely used for analysis, the report says.

“To understand migration flows in any one sector, it is important to view the entire border as a system,” the panel concluded. “Localized increases in border enforcement may simply change where migrants cross without reducing the overall flow in the long run.”

Existing human surveys conducted on both sides of the border hold promise for improving statistics, the report said, including one called the Survey of Migration in the Northern Border of Mexico sponsored by the Mexican government’s Ministry of Labor and Social Welfare and first developed by biologists studying migratory populations. Questioners visit train stations, bus depots and international bridges in border towns and approach people crossing in both directions, including those who have been apprehended by authorities.

The panel’s own work was limited because the Homeland Security Department balked at handing over data on Border Patrol apprehensions after researchers took the position that it should be made publicly available in order for the study to have credibility.

The team argued that the Border Patrol’s concerns could have been addressed by masking sensitive fields of data and using broader geographic areas to describe apprehensions, rather than attributing them to individual Border Patrol stations. Department officials still refused without a promise that the data would be kept secret.

While enough information was available to determine how well migration flows were being estimated, the authors argued that Washington “would greatly benefit from making the administrative data in its enforcement databases … publicly available to the research community.”

It wasn’t the first time the National Academy of Sciences clashed with Homeland Security officials over government transparency. For five years, authors behind another report from the academy quietly battled the department over whether their findings would remain classified and thus beyond public reach. The 2007 report concluded that the nation’s power grid was susceptible to both natural disasters and terrorist attacks, but the Homeland Security Department, which itself sought the academy’s guidance on the issue of electrical power and vulnerability, didn’t agree to release the findings until Nov. 14 of this year.

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G.W. Schulz is a reporter for Reveal, covering security, privacy, technology and criminal justice. Since joining The Center for Investigative Reporting in 2008, he's reported stories for NPR, KQED, Wired.com, The Dallas Morning News, the Chicago Tribune, the San Francisco Chronicle, Mother Jones and more. Prior to that, he wrote for the San Francisco Bay Guardian and was an early contributor to The Chauncey Bailey Project, which won a Tom Renner Award from Investigative Reporters and Editors in 2008. Schulz also has won awards from the California Newspaper Publishers Association and the Society of Professional Journalists’ Northern California Chapter. He graduated from the University of Kansas and is based in Austin, Texas.