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