Judging by city officials' accounts, Eagle Mountain is a bustling community, which saw its electrical hookups grow by 659 from July 1, 2006, to July 1, 2007.

With an average household size of 4.1 in April 2007, the northwestern Utah County community apparently grew by some 2,700 people during that year.

Yet recent census estimates show Eagle Mountain growing by only 441 people from July 1, 2006, to July 1, 2007.

Eagle Mountain isn't alone in its discrepancy with the U.S. Census Bureau. Utah's population trackers have long contested the census population estimates.

The numbers for July 1, 2007, should reflect the tail end of a building boom that has since slowed amid the nationwide housing slump.

"They are quite a bit short on what we believe our population growth was at the time," said Peter Spencer, Eagle Mountain's planning director. He said city officials believe the population is closer to 22,000 than the census estimate of 17,832.

An accurate count is important, Spencer said, because the census estimates are tied to road funding and tax revenues.

Last year, the Utah County city launched a challenge that boosted its 2006 population from an estimated 12,232 to 17,391. A handful of other Wasatch Front cities also challenged their estimates, as did Cache County as a whole. "All we can do is challenge," said Shawn Eliot, transportation planner for the Mountainland Association of Governments.

Eliot said he expects many cities in Utah County will again find their estimates low, particularly given that the census estimated the entire county growth at 12,000 — far short of the 25,000 people the state estimates were added to the county's population.

The city populations must add up to the census' estimate for county populations, which can mean deflated estimates for cities that don't challenge their results, said Pamela Perlich, senior research economist at the University of Utah.

"If you give to one you have to take from another," says Perlich.

She believes that methodology explains Salt Lake City's estimated loss of 1,122 people since 2000, despite new construction of 3,750 new housing units in the city over that time period.

"It makes no sense," Perlich said. "It's a mechanical process, and mechanical processes do generate errors."

The issue becomes even more problematic for counties, such as Utah or Cache, that have large populations of college students, she said, because those counties tend to have underestimated populations as a whole.

That's because the census uses income tax returns to estimate county populations. That methodology misses many college students who are claimed on their parents' taxes, so their movement into the county isn't captured. However, when those students graduate and move to another county to start their careers, they may be counted as having migrated out.

"It's an issue for college counties in general," Perlich said. "It underestimates the in-migration and overestimates the out- migration. ... Utah County has particularly suffered."


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