Relatively few of Utah's undocumented immigrants are school age, an estimate that a University of Utah researcher says would have been difficult to decipher without a new census tool.

"There are not that many kids," said Pamela Perlich, senior research economist for the U.'s Bureau of Business and Economic Research.

An estimated one-sixth to one-seventh of the state's 75,000 to 100,000 undocumented immigrants are school age, Perlich said. She developed that estimate using data from the Pew Hispanic Center. Then she checked that against the Census Bureau's American Community Survey, which shows similar numbers for the overall noncitizen foreign-born population.

Perlich helped legislative auditors with demographics for an audit set to be publicly released May 22. At a news conference Thursday at the governor's office, she used that as just one example of how the annual ACS survey of 250,000 households nationwide is becoming a useful tool in tracking demographic and economic trends.

"It's not just a matter of growth, it's change of composition and characteristics," she said. "Disentangling what is Utah, to how Utah is changing due to the influx has been a challenge."

Having timely data is key to understanding Utah's changing demographics as the minority population, historically about 2 percent, is approaching 20 percent and as the state starts to see the impact of the aging baby boomers, Perlich said.

It's important for information to be up to date, especially in Utah, the nation's sixth-fastest-growing state, so that governments and businesses can make informed planning decisions, said Preston Jay Waite, deputy director of the U.S. Census Bureau.

Waite said beyond apportionment, census data are used, in whole or in part, to distribute $200 billion annually in federal funds. And he said the ACS's annual tracking is helping the federal agency better prepare for the 2010 Census.

"I don't need to tell you ... it's very important to have an accurate and complete count," Waite said. Utah narrowly missed out on a fourth congressional seat following the 2000 Census, when its LDS missionaries serving out-of-state weren't counted.

Demographers have said current trends show Utah easily gaining that fourth seat in 2010, and a bill is pending in Congress that would give Utah the seat a bit early, along with a seat for the District of Columbia.

The continual ACS survey, which contains information such as household income and educational attainment, has been phased in since 2000 as a replacement of the census long form. One of the biggest reasons for the annual survey is the speed at which everything from population counts to employment change in today's world.