SAN DIEGO — Kathy Gomez estimates that U.S. Border Patrol agents catch 75% of the migrants who try to run through the strawberry fields at her farm near the border with Tijuana. Farther east, Miguel Diaz thinks the number hits 90% at his junkyard near the base of Otay Mountain.
But in the San Diego backcountry, rancher Bob Maupin says that, of the migrants who skirt his 250 acres, only 10% get arrested.
Across the Southwest, the rate at which the Border Patrol stops illegal crossings has long been the stuff of coffee shop speculation. In Washington, an effort to make those numbers precise is about to become the thread on which the fate of millions will hang.
Under a bipartisan immigration bill being debated in the Senate, a 13-year path to citizenship for most of the county's 11 million unauthorized immigrants would depend on whether the Border Patrol can achieve 90% effectiveness.
Supporters say they have confidence such a goal can be measured and reached over the next decade. Border residents, some experts and longtime patrol agents express deep skepticism.
How, they ask, can anyone reliably estimate the number of immigrants that agents don't catch? Can anyone create a precise benchmark for so imprecise an effort?
Attempts to come up with accurate figures have long bedeviled the Border Patrol. Although the bill would provide billions of dollars for improved border security, tying immigration reform to a border security "trigger" of 90% almost invites manipulation, say critics like Shawn Moran, vice president of the National Border Patrol Council, the union for patrol agents.
"How are they going to measure effectiveness?" Moran asked. "It will put pressure on Border Patrol management to fudge the number in order to fit political purposes. That's my concern."
The proposed Senate bill would give the government an additional $4.5 billion to add patrol agents, fences, surveillance drones, advanced radar and other surveillance equipment in the first five years after the legislation passes.
The Border Patrol would have five years to demonstrate it can capture or turn back 90% of people illegally attempting to cross the most trafficked areas of the Southwest border. If that deadline isn't met, a commission made up of governors and law enforcement leaders from border states would be created and given an additional $2 billion and another five years to fill the remaining gaps.
Only after those measures are achieved — along with a new system allowing the government to track people leaving the country through airports and seaports and a mandate for employers to use a federal database to verify the immigration status of new hires — would immigrants who had been granted legal status be allowed to apply for permanent residency and eventual citizenship.
Backers of the legislation insist that the combination — tighter enforcement at the border, exit data to track people who overstay their visas and employee verification to forestall illegal hiring — will prevent a new upsurge of illegal immigration.
Sen. John McCain (R-Ariz.) voted for the 1986 bill that gave legal status to 3 million immigrants. Critics of that law say it triggered a second wave of illegal immigration.
"I am not going to stand for a third wave," McCain said. "I am confident that the technology and surveillance capability as well as the drones will allow us to have effective control of the border."
Sen. Charles E. Schumer (D-N.Y.), his negotiating partner in the group of eight senators who put together the immigration deal, told reporters recently that they had insisted "that these metrics not be spaghetti — that they be achievable, concrete metrics."
Supporters of immigration reform hope the additional money will take away a powerful argument from the other side.
"Opponents of immigration have used border security as an excuse," said Ali Noorani, executive director of the National Immigration Forum in Washington, which supports a pathway to citizenship for unauthorized immigrants. "This puts the pressure on them."
Officials within the Obama administration, which backs the bill, doubt that a single, precise number can ever capture the complex reality of the border.
Homeland Security Secretary Janet Napolitano told senators last week that a broad spectrum of statistics, including crime rates and property values as well as enforcement figures, shows that the border is more secure than it has been in four decades. But she counseled senators not to try to sum up border security in a single percentage.
"No one number captures the evolving and extensive nature of the border," Napolitano told the Senate Judiciary Committee during a hearing about the immigration bill. "There's no one metric that's your magic number."
Measuring the effectiveness of the $3.5-billion-a-year Border Patrol has long proved to be an elusive task. Several years ago, the agency rated its performance by assessing how much of the border was under "operational control." Officials determined "control" by measuring the agency's ability to detect, respond to and interdict migrants crossing the border.
By 2010, the agency reported that 44% of the border was under operational control, including about 85% of the section in California. Border Patrol officials focused on the improvement over past years, arguing that the figure showed a steady increase in effectiveness. Critics pounced on the agency's tacit admission that 56% of the border was not under control and said the agency was underperforming.
The agency has since discontinued the use of operational control as its yardstick and now cites migrant arrest totals as a measure of performance. Homeland Security officials, pointing to declining arrest numbers in recent years, repeatedly have said that the border is more secure than ever.
But a 2012 report by the Government Accountability Office, the investigative arm of Congress, said the use of arrest totals did not measure actual results and could lead to reduced oversight and accountability of the agency. Other experts have noted that the poor economy in the U.S. in recent years has discouraged workers from coming across the border. As the economy improves, no one knows whether that trend will quickly turn around.
One consistent problem is that border agents lack a reliable way of counting the number of people they don't see. To remedy that, senators have pointed to a new radar system developed to detect insurgents planting bombs in Afghanistan as one piece of technology that could help track people crossing unlawfully into the United States.
Called Vader, the drone-mounted radar has been tested over a 150-square-mile patch of Arizona desert for more than a year.
Homeland Security officials are also developing a new set of measurements called the "border condition index." Officials say the index will track trends in apprehensions, including the percentage of criminals stopped, where migrants are coming from and whether people are trying to cross again and again.
But the officials have emphasized that the purpose of the new index is to help them decide where to send more agents, not to give an overall score for border security.
Members of Congress have reacted with incredulity to claims that the department cannot come up with a single figure to measure border security.
"Excuse me," Rep. Candice S. Miller (R-Mich.) said at a recent hearing of the House border security panel that she chairs. "I'm just trying to let this all digest here." Without "a high degree of confidence that we are securing our border," she said, "I think this whole comprehensive immigration reform is going to be a very difficult lift."