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
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@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
(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,
816.474.4240
816.701.8237