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/



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?????




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



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.






Frank Lenk

Director of Research Services

Mid-America Regional Council

600 Broadway, Suite 200

Kansas City, MO 64105