A new report reveals that Washington has spent $250 billion in its war to stop illegal immigration, but it remains unclear if it worked in large part because the federal government hasn’t mastered a way to count who sneaked in and who overstayed their visa deadline.

“The federal government has spent more than $250 billion on immigration enforcement, including at the U.S.-Mexico border, over the past three decades, vastly expanding the size of the Border Patrol, building hundreds of miles of fencing, and adding technologies such as ground sensors and drone flights to detect unauthorized entries,” said the report from the Migration Policy Institute.