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Saturday, June 30th, 2012

Misreferencing Misoverestimated Population by Chris Briem

[ Chris Briem works at the Program in Urban and Regional Analysis at the University of Pittsburgh's University Center for Social and Urban Research. He's one the best I know anywhere at urban data, and his personal blog Nullspace is a fantastic, data wonkish take at Pittsburgh. I happened to see this article he wrote there which deflates a lot of the recent enthusiasm created by the latest census population estimates for municipalities. This stuff was news to me, so I'm grateful Chris allowed me to repost it here - Aaron. ]

I know the media confusion story of the day is all about the momentary misreporting that got the story of the Supreme Court ruling yesterday backwards. Yet there was some real misoverestimating across the nation over the latest census numbers that were released yesterday on municipal population estimates for 2011.

Here are some headlines yesterday: 

LATimes: U.S. population in cities growing faster than in suburbs

Chicago Tribune: Census sees Chicago’s population inching up

Boston Herald: U.S. population in cities growing faster than in suburbs, figures show

AP: Big US cities boom as young adults shun suburbs, census estimates show

Lots more just like those. Guess what… Pretty much all of those stories are wrong, or at the very least baseless when you really look at the data.

The census data reported was the 2011 population estimates for incorporated places across the US. So basically cities, towns, boroughs, and townships. We went through this yesterday, but if one read the actual census methodology for this particular data they were quite clear. The subcounty (i.e. municipal) population estimates are mostly based on an estimate of the change in housing units at the municipal level.  The census changed their methodology on how they computed housing unit change for this particular data and as they explain:

To produce subcounty housing unit estimates, we distributed the extrapolated county estimates down to each subcounty area within a county based on 2010 Census proportions.” (emphasis added)

Which means basically that there was very little 2011 data that went into these numbers. Without using new information it begs the question of how much the results should be interpreted. They basically took the estimated county level population data and allocated it to smaller municipalities based on the 2010 Census. They also just assumed that all the growth was even within counties. That assumption, that center cities grew the same as their immediate suburbs, produced the results being reported on everywhere. There appears to be no other supporting analysis for the assumption, it is just an assumption. Other than that, there is no new information here to lead to the conclusions making their way into the headlines.  It may have even tripped up the experts out there because the Census folks explain they changed their methodology just for this particular data release, and are likely to change it again before next year’s update.   But you have to read into their methodology notes to realize the changes for just this year. This is all probably an example of why some of us have the bad habit of reading footnotes first. 

Was there any new growth in cities? Not at all. Or at least there is no data in any of this to tell us one way or another. The Census basically took the growth that likely continued to be mostly in the suburbs and just assumed it was spread evenly between center cities and suburbs within counties across the nation. The result was that it all of a sudden appeared cities were growing faster (or in some cases shrinking less) than they have been in other data. In reality, the new patterns were no more than an artifact of the temporary change in the Census Bureau’s methodology for this data. If they had ever used the same methodology in the past, namely taking county-wide population changes and distributed growth evenly across municipalities the results would have come out the same.  If these municipal estimates had been calculated this way over the last decade, they would have wound up being very much different from the eventual decennial census enumeration.

So the headlines may be ok if there is data on ‘cities’ that are in themselves counties, but those areas are few; or in the case of New York City, multiple counties.  For most cities are only parts of larger counties. Other than Allegheny County I looked at Cook County which includes Chicago and indeed both the city of Chicago and most all of its Cook County suburbs are being reported as having nearly identical growth rates since 2010. I bet that is no more true there than it isn’t here.

The only caveat to any of that is that the data reported does seem to have some new 2011 data on group quarters population incorporated into it, as their methodology says it should. So where there was a recent change in the population of college dorms, military barracks, prisoners are related types of institutions then you are seeing population changes different from the county-wide averages. That appears to me the main source of the disproportionate growth the 2011 data is showing for the City of Pittsburgh. So real growth for sure, but I would be careful in explaining its causes.

So this all may not be as egregious an error as the news cycle we once had in 2000 when  population ‘growth’ Downtown was attributed to a big new influx of young people living in the Golden Triangle in the 1990’s. The truth was that the Allegheny County Jail was rebuilt and expanded in the 1990’s and that expansion more than accounted for a nominal reported increase in Downtown’s residential population. The eventual increase in Downtown’s population would come mostly a decade and several hundred million dollars in subsidies later. Nonetheless, this misuse of Census data is certainly more widespread and likely be misreferenced for years to come.

This post originally appeared in Nullspace on June 29, 2012.

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