President Obama speaks at the Young Southeast Asian Leaders Initiative town hall at Taylor's University in Kuala Lumpur, Malaysia, on Friday. (AP Photo/Vincent Thian)

While filing his paperwork Friday to be on the ballot in New Hampshire, Republican presidential candidate Ben Carson was asked about the topic of the day: Donald Trump's apparent endorsement of a database tracking Muslims in the United States. "Hopefully," Carson said, "we already have a database on every citizen who is already here."

Good news. We do — and it's almost certainly supplemented with the avalanche of data compiled by the private sector.

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If you go to the Facebook ad-buying site — which anyone can do; it's right here — it takes only a few clicks to get the social network's estimate of the number of Americans on Facebook who are interested in the Muslim faith. Cobbling together everyone who lives in the U.S. and who has liked pages or items dealing with Islam gives you 9.4 million possible targets for your ads — a number substantially higher than the 2.75 million Muslims that Pew Research estimates live in the U.S. But still: Facebook has a sense of who's interested in the subject.

If that level of pinpointing surprises you, wait for the punchline. Facebook's information is very, very rough compared to that compiled by large consumer data companies. According to an expert who spoke with The Fix, it is also almost certainly the case that the government buys that data to get a clearer picture of the American population.

Here's how it works. Go to the Google ads site and you can see what the search giant thinks it knows about you — or, at least, what it's willing to share about what it knows. Based on my search history and the Web sites I visit, Google figures that I am a male age 35 to 44, which is correct.

Google knows a lot about me, but not everything. Imagine if you combined what Google knows about me with what my grocery store loyalty card knows about me and what my cable provider knows about me. It's the blind-men-examine-an-elephant situation: Each company sees only a small part, and so it doesn't know me well. By giving information to a central source, a data broker such as Acxiom or Experian, marketers know that I'm an elephant. Or a donkey. Or anything else.

"The data broker industry is sophisticated enough to pick out people as if they were grains of sand on a huge beach, identify them with particularity and make fine-grained determinations about virtually everything in their lives," said Tim Sparapani, principal of SPQR Strategies and former senior legislative counsel for the American Civil Liberties Union who, in that capacity, testified before Congress on the subject.

The brokers learn who you are "by amalgamating data from people's offline lives and their online lives," he said. Data such as credit card purchases, browser history, online shopping, who you donate to, magazine and newspaper subscriptions, purchases in grocery stores where you swipe your loyalty card. Particularly for credit card purchases, "it's really trivially easy for that data to be sifted and sorted," Sparapani said. All of that data isn't always specific to an individual (whose Time magazine subscription is it, anyway?), but data can also easily build out a profile of a household. Where you vacation. Where you work. They can analyze languages spoken and last names to develop a picture of a person's or family's ethnicity, or religion. It goes on.


From Experian's "Ethnic Insight" audience guide. (Experian.com)

Where does this overlap with the government? The government could create a tool to pull in data from all of these other places, too. But why bother, when the private sector already has? "I think it's a well-established fact that the government, writ large, is the largest source of funds for the data brokerage industry," Sparapani said. "They have elastic budgets. They can spend whatever they think they need to spend, particularly post-9/11 and in an era of ISIS commanding our attention." Many of those budgets are classified.

You know of the government's existing databases instinctively: Social Security, Census data, your college or home loans, your tax information, and so on. For immigrants and refugees, there's more information, including interviews and background checks. Law enforcement — the FBI, Homeland Security — wants to identify the people they should be keeping an eye on. So they take the government's existing data and overlay the data compiled by the private sector. Then match that against the the profile of who you think is a threat. It's almost certain that if the government wanted to pick out the Muslim population in an area, they could do so quickly and accurately.

We all know about one of these databases: the government's no-fly list. This is a more-constrained version of the Muslim database that's been discussed this week on the campaign trail (though it's not that constrained). It is a list of possible threats that the government and parts of the private sector (such as airlines) can access. Obviously, there are also other names that were considered and screened for the list and not added. The government also has a larger "terrorism watch list," which exists for precisely the reason that you'd expect.

In other words, there's no need for Donald Trump or anyone else to start registering Muslims at mosques (as Trump suggested he might do in response to a question on Thursday) to create a picture of the population.

"It's absolutely unnecessary for the U.S. government to build these lists," Sparapani said, "because the DHS and others have already purchased these materials — and they do every day — from the data brokerage community."