New Census data released Thursday show that just less than half the population growth in Allegheny County occurred within the city proper, largely due to an estimated increase in Pittsburgh's dormitory and jail populations.
Census estimates revealed that between April 1, 2010, and July 1, 2011, 3,718 people moved into the county. The city's population increased by an estimated 1,780 over that same 15-month period to 307,484.
About 1,200 -- or 70 percent -- of those new Pittsburgh residents settled in city group quarter arrangements, said Rodger Johnson, chief of the bureau's Local Government Estimates and Migration Processing Branch.
The bureau defines group quarters as college dorms, military barracks, jails, group homes and other facilities owned or managed by an entity providing housing or other services.
"The growth that we're seeing in Pittsburgh is primarily dormitory and jail and prison-related type growth," Mr. Johnson said.
The state Census team at the Pennsylvania State Data Center at Penn State's Harrisburg branch provided numbers of group quarters for this latest estimate. That also explains why growth in county municipalities was evenly distributed for the most part, Mr. Johnson said.
Every July, the Census Bureau estimates the population of U.S. cities. The methodology behind the latest data set is different this year though, Mr. Johnson explained, because the bureau didn't have residential building permit information available, which they've used to measure changes in the past.
Instead, Census teams examined the housing change for the nine months prior to the 2010 Census, projected that forward to July 2011 and "broadcast that change down" to the county's municipalities, he said.
Christopher Briem, a regional economist for the University of Pittsburgh's University Center for Social and Urban Research cautioned against reading too much into this set of population estimates.
"These numbers don't represent what they've normally done with estimating city populations," he said. "So it's not just dorm and jail populations -- I think the remainder of the growth itself is something you might question in the future."
The Census Bureau is evaluating its earlier method to measure population estimates but expects to restore parts of it for the 2012 count.
"Next year's numbers for Pittsburgh and other areas may look a little different," Mr. Johnson said.
Molly Born: firstname.lastname@example.org or 412-263-1944. First Published July 2, 2012 12:00 AM