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?????
[mailto:firstname.lastname@example.org] On Behalf Of Michael Cline
Sent: Tuesday, April 21, 2009 8:25 AM
Subject: RE: [CTPP] Comparison of ACS 1-year, ACS 3-year & Annual
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
Institute for Demographic & Socioeconomic Research
The University of Texas at San Antonio
1 UTSA Circle
San Antonio, TX 78249-0704
[mailto:email@example.com] On Behalf Of Frank Lenk
Sent: Monday, April 20, 2009 11:26 PM
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.
Director of Research Services
Mid-America Regional Council
600 Broadway, Suite 200
Kansas City, MO 64105