In the wake of an Arizona law allowing police to detain anyone suspected of being in the country illegally, lawmakers in many states, including Idaho, are looking more closely at illegal immigration — even with that Arizona law recently put on hold by a judge.

But quantifiable data is hard to find, and most of the numbers bandied about are based on research by advocacy groups on either side of the debate. That makes it difficult to know both the costs of illegal workers and the benefits of a low-cost, mobile work force.

An Idaho Statesman survey of several Treasure Valley institutions shows that many of them — hospitals, school districts, universities and state agencies — have no mechanism to track the costs and benefits of undocumented workers.

This is not an issue the Idaho controller's office has studied, but many of these costs are difficult to quantify, said Chief Deputy Dan Goicoechea.

Even knowing how many illegal immigrants live in Idaho is nearly impossible.

The Pew Hispanic Center estimated that in 2008 between 25,000 and 40,000 illegal immigrants lived in Idaho — among the lowest in the nation. But the key word is "estimated."

"I challenge anyone to give us the data on the undocumented. We just don't know," said Margie Gonzales, executive director of the Idaho Hispanic Commission.

Complicating the statistics is the Constitution, which grants citizenship to any child born in the United States, including the offspring of illegal immigrants.

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