Wednesday, June 21, 2006

Ever shrinking city......

So I feel obliged to respond a bit to the news blurbs today (such as these in thePG, Trib, and KDKA) about some new census data showing continued decline in the city's population. At least two points need to be addressed.

One is that these are just estimates based on estimates. The municipal level data is just a refinement of county level estimates reported months ago. The allocation of growth at the municipality level is mostly based on the spatial pattern of private residential building permits through 2004. Using building permits as a proxy for growth makes a lot of sense (people need to live somewhere so you get correlation) but there are a lot of other things going on. It's easy to overinterpret year over year, of town by town, differences. The only other major adjustment to the model is that it accounts for depreciation of current housing and guess what, Pittsburgh has a really old housing stock which means the model assumes a larger proportion of it goes out of service each year than in cities with more modern housing. Less housing means fewer people imputed as living in the city proper.

Two is whether this was a city story at all. The headline factoid everywhere is that the city of Pittsburgh lost 1.3% population between 2004 and 2005. Even if that number is right, is this reflective of some bad mojo only in the City?? Well, in the same data you can see that in fact the City proper compares relatively favorably to most other municipalites in Allegheny County. Here is my list showing that 80 of the 128 municipalities in the county had faster decline than the City proper. No mention of that anywhere?? People moving into the suburbs, and even farther into the exurbs, is a national phenomenon and surely not anything unique to Pittsburgh. Pittsburgh is a relatively small city, which means it is much more made up of the inner urban core that is being hit the hardest in all regions.

In this data Cresecent Township grew by an amazing 9.3% between 2004 and 2005. Did it really and if so why? Well... in 2004 there were building permits for 96 single family homes for the township, more than 10 times the average of 9 per year over the previous 5 years. Thus the model allocated a lot of growth to that one municipality this year. Looking forward, it is an easy prediction that next years number for the city of Pittsburgh will be worse than this year. Consider that next year's population estimate will be based on the number of building permits in 2005. The census shows that only 65 permits for private residental single family homes were issued for the city in 2005, the lowest number in a decade.

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