Getting a Handle on 'Fuzzy' Immigration Numbers Efforts to estimate the number of illegal immigrants living in the United States can be tricky. To begin with, any estimate is obviously a snapshot in time. The current estimated range is between 8 million and 20 million people, though almost all reputable sources believe the 8 million mark was surpassed several years ago. The number most commonly cited is between 11 million and 12 million.

# Getting a Handle on 'Fuzzy' Immigration Numbers

Efforts to estimate the number of illegal immigrants living in the United States can be tricky. To begin with, any estimate is obviously a snapshot in time. The current estimated range is between 8 million and 20 million people, though almost all reputable sources believe the 8 million mark was surpassed several years ago. The number most commonly cited is between 11 million and 12 million.

That estimate is derived by using what's called the statistical "residual method. Here's how it works:

1 -- Take the number of legal immigrants: about 24-25 million. They are relatively easy to track because they have documents such as green cards.

2 -- Take the total number of foreign-born persons in the United States: about 35-36-million. That number includes legal immigrants, but it also accounts for other immigrants, as statistically sampled in periodic U.S. Census Bureau surveys (those are interviews done to update the 10-year Census).

3 -- Subtract the legal immigrants from the total number of foreign-born immigrants. Illegal immigrants equal roughly 11 million to 12 million.

Demographer Jeff Passell at the Pew Hispanic Center says illegal immigrants are surprisingly forthcoming when interviewed anonymously. Plus, the surveys use statistically valid sampling methods.

An Accurate Picture?

Other sources, such as investigative journalist Donald Barlett, believe the number is an underestimate, because illegal immigrants are wary of authorities and tough to track down.

Barlett cites a report by the investment firm Bear Stearns that estimates as many as 20 million illegal immigrants as of last fall. That report uses micro-economic trends such as housing starts, school population forecasts, and soaring remittances--the amount of money sent back specifically to Mexico.

What Border Patrol Apprehensions Tell Us

The federal government cites the number of Border Patrol apprehensions to gauge the success of current border-enforcement strategy. Here's how the numbers are derived:

When illegal immigrants are caught, they are taken to a processing center, where they are photographed and fingerprinted. Those with criminal records and those who admit they are not from Mexico are detained. The "OTMs" (other than Mexicans) are detained until the can be returned, usually via airplane, to their home country.

Many illegal immigrants are released on their own recognizance, pending a court date. They often disappear into cities. However, the overwhelming majority of those apprehended are Mexican; they are put on a bus and driven to a port of entry on the Mexican border and released to walk a few feet back into Mexico.

Since many of these illegal immigrants come from the deep interior of Mexico, once they are dropped off at the border, they will often try again to enter the United States without authorization. (Some are flown back to Mexico City, but that's a voluntary program.) Immigrants I've interviewed often tell me they had to try two, three, four times or more to enter the U.S. -- and eventually they made it in.

Double Counting?

The problem is, each time an illegal immigrant is caught, the Border Patrol counts that person as an apprehension. Same person, three apprehensions -- sometimes on the same day. Even the Border Patrol says three people enter the United States illegally for every one person who is caught trying to do so.

Critics such as Douglas Massey, a Princeton University sociologist and expert on Mexican migration, say the apprehension figure is misleading.

"There is virtually nothing one can infer about the volume of illegal migration from the number of apprehensions," Massey told the Arizona Daily Star.

Because those apprehended are photographed and fingerprinted, it seems likely the federal government has statistics -- or could compile them -- on the number of distinct individuals it catches. But the government has not released those figures for several years. Meanwhile, a number of media outlets continue to use "apprehensions" as interchangeable with "permanently removed."

Immigration vs. Migration

Historically, people have migrated from Mexico illegally for seasonal work, especially in agriculture. No agency tracks those who migrate illegally, but they are not "immigrants" because they do not stay in the United States. Instead, these migrants moved back and forth across the border.

But border security has increased both the risks and the cost of that circular migration. As a result, there is anecdotal evidence that more people are staying in the United States illegally and even sending for their families. But, again, no numbers yet exist that would accurately prove or disprove that theory.