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(a)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(a)chrispy.net
[mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Frank Lenk
Sent: Tuesday, April 21, 2009 9:10 AM
To: ctpp-news(a)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(a)chrispy.net
[mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Michael Cline
Sent: Tuesday, April 21, 2009 8:25 AM
To: ctpp-news(a)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(a)utsa.edu <mailto:michael.cline@utsa.edu>
http://idser.utsa.edu <http://idser.utsa.edu/>
From: ctpp-news-bounces(a)chrispy.net
[mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Frank Lenk
Sent: Monday, April 20, 2009 11:26 PM
To: ctpp-news(a)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(a)marc.org <mailto:flenk@marc.org>
816.701.8237
------------------------------------------------------------------------
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