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 816.474.4240 flenk(a)marc.org 816.701.8237