17 Aug
2006
17 Aug
'06
10:33 a.m.
TO: CTPP-News Listserv
FR: Chuck Purvis
This is an email I prepared for our Bay Area and California Census
listservs, regarding newly released ACS data.
PLEASE, if there are other metro or state agencies who've analyzed the
ACS household population relative to their independent estimates of
household population, PLEASE share your research!
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TO: Bay Area Census Listserv
FR: Chuck Purvis, MTC
The "first wave" of the American Community Survey data for 2005 was
released yesterday, August 15th. This first year of the full ACS
database EXCLUDES group quarters population, so any of the "TOTAL
POPULATION" tables as shown by the Census Bureau's American FactFinder
are actually "HOUSEHOLD POPULATION" (or population in households,
whichever term you prefer.) The data for total population that INCLUDES
group quarters will be included in the 2006 ACS data, to be released
summer '07.
Over the past year we have had concerns that the household population
in the ACS (the 2004 ACS, to be particular), was significantly lower
than decennial census counts of household population, and households.
This is now a real concern with the 2005 ACS, as well. What we are able
to compare for California is the household population estimates from the
2005 ACS, to our State Department of Finance's (DOF) estimates of total
population, household population, GQ population, housing units, occupied
housing units, at the city and county level, from their "E-5" estimate
reports, available on the DOF web site
(http://www.dof.ca.gov/HTML/DEMOGRAP/Druhpar.asp). There are standard
errors in the DOF data, as suggested in their methodology, but DOF
doesn't provide explicit confidence intervals around their estimates, at
least as far as I could tell.
So, I've been able to analyze the data for the 119 California Places
(116 cities + 3 census designated places); and the 40 California
Counties, that exceed the 65,000 household population threshhold for
reporting ACS totals. I've also created tables for the San Francisco Bay
Area Cities (25 cities) & Counties (9 counties) in the same EXCEL
workbook. This EXCEL workbook is available from the MTC web site, at:
ftp://ftp.abag.ca.gov/pub/mtc/planning/ACS/
Of the 119 California Places with household population estimates
exceeding 65,000, thirteen of these cities are significantly different,
comparing the 1/1/05 DOF (Department of Finance) household population,
and the 1/1/06 DOF household population estimates, to the 95 percent
confidence interval around the ACS household population estimates.
Unfortunately this list includes six of the ten largest California
Cities:
1. Los Angeles: ACS = 3,668 to 3,794 thousand; DOF = 3,852 to 3,892
thousand (estimates for 1/1/05 and 1/1/06)
2. San Diego: ACS = 1,176 to 1,240 thousand; DOF = 1,254 to 1,263
thousand
3. San Jose: ACS = 866 to 908 thousand; DOF = 931 to 943 thousand
4. San Francisco: ACS = 699 to 739 thousand (estimated); DOF = 773 to
778 thousand
8. Oakland: ACS = 355 to 393 thousand; DOF = 403 to 404 thousand
10. Santa Ana: ACS = 277 to 328 thousand; DOF = 345 to 346 thousand.
The other seven California places with a significant difference,
comparing ACS to our DOF, include: Garden Grove (Orange County); Salinas
(Monterey County); Pasadena (LA County); Daly City (San Mateo County);
Berkeley (Alameda County); Alhambra (LA County); and Merced (Merced
County).
A critical check in the coming months will be to compare the ACS
estimates of housing units and households (occupied housing units) to
independent estimates of housing units and households, also available
from the DOF Report E-5. Perhaps the decline in population in these
large California cities can be attributed to decreasing average
household size. On the other hand, it is a lot harder to explain any
absolute losses in housing units within a community. It may be useful
for the City Planners in Large Cities to have at hand independent
estimates of new housing units constructed and old housing units
demolished over the 2000 to 2005 time period.
These are very serious concerns about the ACS. I believe it has more to
do with the "sample frame" and weighting/expansion issues, as opposed to
the quality of the characteristics derived from the ACS, but this is an
important topic that should be discussed by our state data center
network, the Census Bureau and interested stakeholders.
Hope this helps,
Chuck Purvis
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Charles L. Purvis, AICP
Principal Transportation Planner/Analyst
Metropolitan Transportation Commission
101 Eighth Street
Oakland, CA 94607-4700
(510) 817-5755 (office) [new, 8/1/05]
(510) 817-7848 (fax) [new, 8/1/05]
www: http://www.mtc.ca.gov/
Census WWW: http://www.bayareacensus.ca.gov/
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