Thursday, April 03, 2014

A number is a number is a number

So I hate to do this.... but worth deconstructing a recent viral blog post that obviously has made great note here in Pittsburgh.  See this: The Nation's Most Coveted Demographic Has Been Flocking To Pittsburgh. The headline  comes from a straightforward compilation of demographic data from the 2005 American Community Survey (ACS) and comparing with 2012 data from the same source. Census data is census data; so what could be wrong?

But yeah, I hate to say it, but probably does not work out that way.  A number of things going on here, but first and foremost I refer back to my post trying to explain how the American Community Survey is not intended to be a reference for count data. This may be a good example of why.

That in itself is not the issue here.  In this analysis the population change between 2005 and 2012 was compared, using ACS data to make that comparison.  The thing that was a big red flag for me was the result showing that the Pittsburgh MSA had grown by 2% over the 7 years. I recreated the data and indeed if you just pull up 2005 and 2012 1-year ACS total population counts for the Pittsburgh MSA you get 2,314,937 and 2,360,733 respectively. Works out to a +1.97% gain, so 2% as reported.

The problem is no other data says there has been metro growth here over that period.  Decline may have abated in just the last couple of years, but there has not been net +2% growth.  So what is going on, and how does it relate to the result that shows this disproportionate growth in our 'young' population.  As much as I would love to drive the silver stake into Border Guard Bob, this isn't going to do it.

Ready?  The 2005 ACS data did not include ANY group quarters population.  To be precise, here is the Census Bureau's disclaimer on the 2005 data:
Data are limited to the household population and exclude the population living in institutions, college dormitories, and other 
So how big is the group quarters population in the Pittsburgh MSA?  In 2010 you are talking 62,679K people, or roughly 2.7% of the region's population.  So if you net that out, or add it in, you get back to realizing the population change for the region was a decline of something less than 1%, but certainly not +2%.  So from there you can't really make any of the follow-on conclusions about young people moving into the region.

Then there was the bigger point to come out of that which was that population 25-34 jumped 12% over the same 7 years. So I recreated that and indeed the population age 25-34 appears to increase from 257,715 in 2005 to 289,617 in 2012 according to the 1 year ACS estimates. Works out to +12.4%.  But again, the earlier number does not include any group quarters. How much of the +31,905 increase in that age range are coming from group quarters?  In itself about 5K. Take that out and it still works out to a palpable increase (~+10%) in the age group.  Still some other things going on, and you lose half of the increase if you use latest 5-year ACS data. Basically the range of variation of in the various ACS estimates really makes it hard to conclude there is much increase at all in that population, certainly nothing close to +12%, but I'll go through that in more detail later. Nonetheless, I bet I will be forced to refute the +12% factoid for long into the future.


Blogger Pete Saunders said...

Chris, thanks for taking the time to critique my post on 25-34 net migration in Rust Belt cities. I was aware that there were ACS data flaws, particularly pre-2010, and thought I'd made a strong enough mention that the data isn't always an apples-to-apples comparison. But I appreciate your showing where the discrepancies were.

Also, when this was posted on my blog it was called Playing With Numbers, Rust Belt Millennials Edition ( I think that was a more accurate depiction of what I was trying to get at than the title Business Insider decided to run, highlighting Pittsburgh. I had no choice in that.

Your blog comes highly recommended by fellow blogger Jim Russell, so I look forward to gaining more insight into Pittsburgh from your perspective. Thanks for setting me straight!

Sunday, April 06, 2014 3:17:00 PM  
Blogger C. Briem said...

I do apologize if that was harsh, and the third party headline was unfortunate.

But you wills see the echo chamber has already begun quoting the result for Pittsburgh:

Wednesday, April 09, 2014 9:00:00 AM  

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