[CTPP] Comparison of ACS 1-year, ACS 3-year & Annual Estimates

Tue Apr 21 14:30:08 CDT 2009

I am not an expert on the ACS by any means, but it appeared to me that 
the 2005-2007 ACS estimates differed the most from the census estimates 
where there were major differences in household size and/or occupancy. 
Thus, one of our towns (Jacksonville AR), has seen a drop in both 
household size and occupancy since 2000 and also shows population 
decline in the 2005-2007 ACS, while the census estimates through the 
same period show slight population growth in Jacksonville.

Also, I want to add that we had four special censuses in cities in our 
area around mid-decade, and the population, household size and occupancy 
figures are not too far from the 2005-2007 ACS results. I still believe, 
though, that both ACS and census estimates still tend to under-count 
population slightly, at least in my region, compared with census counts.

To me it looks like ACS 2005-2007 population totals are affected by 
household size and occupancy. But the correlations I'm seeing could be a 
coincidence. If anyone knows better, please weigh in.

Jonathan Lupton AICP
Research Planner
Metroplan
Little Rock AR
501-372-3300

Elaine.Murakami at dot.gov wrote:

> I contacted Freddy Navarro (Assistant Division Chief for ACS 
> Statistical Design) at the Census Bureau.
>
>  
>
> Freddy said:  "We use the most current vintage (of population 
> estimates) and take an average of the pop estimates and then control 
> the ACS data to those averages.  So, for example, for the 2005 - 2007 
> ACS 3-year estimates, we calculate an average of the 2005, 2006, and 
> 2007 pop estimates using the 2007 series.  That's why comparing the 
> 2007 1-year estimates to the 2005 - 2007 3-year estimates is not an 
> apple to apple comparison.  Such comparisons are discouraged."
>
>  
>
> Elaine's note:  the phrase "current vintage" is important.  For 
> example,  let's say that for County Z, in 2005, the 2005  population 
> estimate made in 2006 might be 100,000.  But, the 2005 estimate for 
> County Z made in 2007 might be 110,000. 
>
>  
>
> The Census Bureau will be releasing an updated version of the Design 
> and Methodology paper in the next few days.  This document includes a 
> section that explains the multiyear weighting methodology in detail.  
> I will assume that it will be posted on this page, on the right side 
> called "documentation"  http://www.census.gov/acs/www/Products/
>
>  
>
> Elaine
>
> ------------------------------------------------------------------------
>
> From: ctpp-news-bounces at chrispy.net 
> [mailto:ctpp-news-bounces at chrispy.net] On Behalf Of Frank Lenk
> Sent: Tuesday, April 21, 2009 9:10 AM
> To: ctpp-news at chrispy.net
> Subject: RE: [CTPP] Comparison of ACS 1-year, ACS 3-year & Annual 
> Estimates
>
>  
>
> After more investigation, it appears that the 3-year estimates are, in 
> fact, controlled to a moving average centered on the middle year. But 
> you have to use the right 3-year estimates. When we did this with the 
> most recent estimates (which now go to 2008), the three-year average 
> centered on 2006 did NOT match, but when we looked at the historical 
> release of 2007 estimates, and used those to calculate an 3-year 
> average, the result DID match the 3-year ACS total.
>
>  
>
> I still find it confusing that the characteristics are NOT averaged, 
> but the totals are?????
>
>  
>
> Frank
>
>  
>
> From: ctpp-news-bounces at chrispy.net 
> [mailto:ctpp-news-bounces at chrispy.net] On Behalf Of Michael Cline
> Sent: Tuesday, April 21, 2009 8:25 AM
> To: ctpp-news at chrispy.net
> Subject: RE: [CTPP] Comparison of ACS 1-year, ACS 3-year & Annual 
> Estimates
>
>  
>
> Someone from the ACS discussed this in January at TRB (at the Census 
> subcommittee meeting).  Unfortunately, I cannot find my notes, but I 
> seem to recall that the 3-year estimates use a middle population 
> estimate (i.e. 2006). 
>
>  
>
>  
>
>  
>
> Michael E. Cline
> Research Associate
> Institute for Demographic & Socioeconomic Research
> The University of Texas at San Antonio
> 1 UTSA Circle
> JPL 4.03.18A
> San Antonio, TX 78249-0704
> (210)458-6537 f(210)458-6541
> michael.cline at utsa.edu <mailto:michael.cline at utsa.edu>
> http://idser.utsa.edu <http://idser.utsa.edu/>
>
>  
>
>  
>
> From: ctpp-news-bounces at chrispy.net 
> [mailto:ctpp-news-bounces at chrispy.net] On Behalf Of Frank Lenk
> Sent: Monday, April 20, 2009 11:26 PM
> To: ctpp-news at chrispy.net
> Subject: [CTPP] Comparison of ACS 1-year, ACS 3-year & Annual Estimates
>
>  
>
> The attached spreadsheet was prepared by my staff, showing the 
> comparison in total county-level population between the 2007 ACS 
> 1-year estimates, the 2007 ACS 3-year estimates and the Census 
> Bureau's July 1, 2007 population estimates by county.  The latter are 
> supposed to be the official population estimates to which ACS is 
> controlled.  And, based on the attached spreadsheet, this appears to 
> be true for (most) counties in the 1-year estimates.  But the total 
> population in the 3-year ACS estimates is systematically biased 
> downwards from the total population in the  1-year ACS estimates 
> and/or the official estimates. 
>
>  
>
> Does anyone have a good idea why?
>
>  
>
> There is some vague language about differences in weighting in the 
> Census Bureau's documentation, but I can't find a satisfying 
> explanation. I do notice that the faster a county is growing the 
> bigger the discrepancy between the 3-year and 1-year estimates of 
> total population. This suggests that the 3-year estimates are being 
> controlled to an average of the 3 years of official total population 
> estimates (2005, 2006 and 2007).  But my understanding is that the3- 
> year ACS estimates are not averaged.  Instead, they a represent a 
> single sample taken over a 3-year period.  My expectation, then, is 
> that this sample would be expanded to the same population as the 
> 1-year estimates - The 3-year and 1-year estimates are, after all, 
> identified by the same year (2007) while a 3-year estimate based on a 
> 3-year moving average would be closer to 2006's 1-year estimate. 
>
>  
>
> Any help in clarifying this issue would be greatly appreciated.
>
>  
>
> Sincerely,
>
>  
>
> Frank
>
>  
>
> Frank Lenk
>
> Director of Research Services
>
> Mid-America Regional Council
>
> 600 Broadway, Suite 200
>
> Kansas City, MO 64105
>
> www.marc.org <http://www.marc.org>
>
> 816.474.4240
>
> flenk at marc.org <mailto:flenk at marc.org>
>
> 816.701.8237
>
>  
>
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