It works like this:
Census ACS three-year estimates are period-based rather than
representing a point in time, with a small number of samples drawn each month over
a 36-month period. Of course, when you have samples, they must be weighted
to represent a control total. That control total is the middle year of
the period. So far, Census has released one-year data for 2005
(households only, no group quarters), 2006, 2007, and the 3-year set for
2005-2007. There is no ACS sample survey data yet for 2008 – only
the annual control totals for each county, the Annual Estimates. The 2008
ACS sample survey data will come out this fall. Since there is no group
quarters data for 2005, the three-year set has its GQ data from 2006 and 2007 weighted
higher to account for the missing year.
Here’s another complication: every year, Census revises the
post-censal annual estimates/control totals for each previous year going back
to the most recent decennial census, but not the ACS datasets controlled to
them. Example: the 2006 ACS sample data is controlled to the 2006 Annual
Estimate, both published in 2007. That 2006 Annual Estimate was updated
and changed in 2008, while the 2006 ACS data remained the same. I assume
the 2005-2007 ACS 3-year dataset (published in 2008) will have been controlled
to the 2008 version of the 2006 Annual Estimate, not the original 2007 version.
But I haven’t actually checked that. If you want consistency
between the ACS datasets and the constantly changing Annual Estimate control
totals, you will have to adjust the ACS figures yourself. What it all
comes down to, is that the ACS figures are best used for data on proportions
(e.g., percent SOV commuters) rather than numbers (e.g., number of SOV
commuters).
Since the ACS data is a sample of only about 2.3%-2.5% (depending
on how you want to count non-respondents), there is a pretty high margin of
error for small universes. If you use the 3-year dataset, it has the
equivalent of about a 7%-7.5% sample size if you assume a stable population (which
of course is never really true), so it offers a smaller margin of error in
exchange for less timeliness. There are a series of published studies on
the Census website addressing the pros and cons of the shift from the decennial
long form point-in-time data to the annual ACS period data.
Pete Swensson, Senior Planner
Thurston Regional Planning Council
2424 Heritage Ct. SW
Olympia, WA 98502
(360) 741-2530 (direct line)
(360) 956-7575 (main desk)
(360) 956-7815 (fax)
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From:
ctpp-news-bounces@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On
Behalf Of Frank Lenk
Sent: Tuesday, April 21, 2009 9:10 AM
To: ctpp-news@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@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On
Behalf Of Michael Cline
Sent: Tuesday, April 21, 2009 8:25 AM
To: ctpp-news@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@utsa.edu
http://idser.utsa.edu
From:
ctpp-news-bounces@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On
Behalf Of Frank Lenk
Sent: Monday, April 20, 2009 11:26 PM
To: ctpp-news@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
816.474.4240
816.701.8237