“Where did everyone go?”: why some population estimates are so far off

I have wonkily tracked population data for many years now, so my eye was caught by an article in today’s Atlanta Journal-Constitution: “Census shock result of flawed estimates”.

“How do 121,000 people disappear into thin air?” asks in the opening line. It’s a reference to the city of Atlanta’s official census count of 420,000 after the 2009 estimate of 541,000. Here in Savannah, the city’s final count was similar to census estimates, but was far below the working assumption of city officials. I wrote about that issue recently in a blog post here, which contains a link to a recent City Talk column.

This issue has a long history here in Savannah. Years ago, when I first started calling into question the studies showing the city’s population was booming, officials countered that their estimates were based on a more dynamic understanding of the data, including the sharp rise in housing permits. But as the AJC piece today outlines, there are multiple problems with the methodologies employed by local municipalities who are doing their own estimates, especially since those government officials are pretty much always trying to boost their population counts.

The AJC discusses four fallacies in today’s piece:
Fallacy No. 1: Building permits=buildings=people
Fallacy No. 2: Household size is static
Fallacy No. 3: Vacancy rates are static
Fallacy No. 4: Tax returns show all migration

The overestimates of Savannah’s population were primarily because of #1 and #3. Housing permits do not always result in units being built or occupied. And vacancy rates ballooned during the recession. Atlanta saw a significant decline in household size between 2000 and 2010, but that seems due to an influx of young single people. According to the census data, Savannah’s household size remained pretty much unchanged over the last decade.

I am writing about this subject yet again for a couple of reasons. As something of an amateur demographer, I’m interested in it, but there’s another reason of more relevance: an assumption of fast growth played a role in a variety of bad decisions by the Savannah city government and by developers within the city limits. Moving forward, we need to get a better handle on growth.