The September version of CTPP "Status Report" newsletter is now posted
at http://www.TRBcensus.com/
The direct link to the newsletter is
http://www.TRBcensus.com/newsltr/sr0906.pdf
--
Ed Christopher
Resource Center Planning Team
Federal Highway Administration
19900 Governors Drive
Olympia Fields, Illinois 60461
708-283-3534 (V) 708-574-8131 (cell)
708-283-3501 (F)
Hello:
In late August 2006, the US Census Bureau (CB) released the 2005
American Community Survey (ACS) data on population, employed persons,
journey-to-work, and many other demographic and employment
characteristics such as income, poverty, employment status, occupation,
and industry. These data are the first from the full implementation of
ACS. This email reviews 2005 ACS data on population and journey-to-work
for the Delaware Valley region which consists of five counties in
Pennsylvania and four in New Jersey.
Population Estimates
The 2005 ACS population estimates are limited to household population
only; they exclude group quarters population. To obtain 2005 ACS
population, the CB estimated group quarters population and subtracted it
from the 2005 total population estimates produced as part of the CB's
Population Estimates Program (PEP). A comparison of 2005 CB population
estimates with those estimated by DVRPC shows that the CB estimates are
very close to DVRPC estimates. The differences between the two sets of
estimates for six counties are less than 2.5 percent. The CB estimates
for two suburban counties are overestimated slightly and one
(Philadelphia) is underestimated, compared to DVRPC's estimates.
An analysis of the 2005 census estimates of group quarters population
shows that none of the nine estimates is reasonable. Group quarters
population are either overestimated or underestimated. For example,
group quarters population for Camden County did not change even by one
person between 2000 and 2005. The Montgomery County estimate decreased
from 23,257 to 22,837 persons rather than increased in this growing
county.
Although the magnitude of group quarters population is generally small,
it increased the margin of error in the ACS household population
significantly. Also, the erroneous group quarters population increased
the margin of error in the 2005 ACS economic characteristics, such as
the number of workers and means of transportation to work. Affected the
most are counties with large group quarters population such as
Montgomery County and Philadelphia.
Journey-to-Work
A review of the 2005 ACS journey-to-work by means of transportation
indicates that the margin of error in the estimates is very large.
Also, the CB could not develop all estimates for Gloucester County, New
Jersey (277,000 people) because the "Number of sample cases is too
small." As is known in Statistics, the sampling error increases
inversely with the size of the variable. Since most people drive to
work, the margin of error in drive alone mode is small. The opposite is
true for those who walk to work. Nine out of 17 means of transportation
are either underestimated or overestimated in Philadelphia as the
difference between 2005 and 2000 estimates is larger than 20 percent.
There is no clear pattern in the 2005 estimates of workers by means of
transportation, some increase and others decrease. Also, there is no
logical reason for the increase or decrease in the number of commuters
that use a particular travel mode. For example, the number of workers
in the DVRPC region that commute in 3-person carpool increases while
those using 4-person carpool decreases. Significant increases and
decreases in the means of transportation to work resulted from using a
small sample of households which does not include group quarters
population.
Average Travel Time
The 2005 ACS commuter travel time in the region is 28.3 minutes. This
value indicates that the average commuter time has declined slightly
between 2000 and 2005 (28.6 vs 28.3). It is interesting to note that
the 2004 ACS travel time increased between 2000 and 2004 (28.6 vs 29.1
minutes). Based on this information, some people may conclude
incorrectly that commuters' travel time increased from 2000 to 2004 and
deceased from 2004 to 2005. No conclusions can be drawn based on this
information since the differences between numbers are small and within
the margin of error in the data. Also, the 2000, 2004, and 2005 census
data are not comparable. They are oranges, apples, and lemons. The
sample sizes are different and ACS data exclude group quarters
population. Although the 2005 ACS sample size is larger than the 2004
sample, it is still about 12 percent of that used in Census 2000.
As I have stated last year after the release of 2004 ACS data, the
errors in the ACS estimates are large and the data cannot be evaluated
or corrected easily. There are no accurate data between decennial
Censuses. As Chuck Parvis stated correctly in his analysis of 2005 ACS
data, it is not possible to "Calculate whether or not there is a
meaningful difference between 2000 and 2005....that's more or less a
professional judgment call."
In summary, the 2005 ACS data cannot be used for transportation
planning. The CB simply connot produce quality data from the ACS
program comparable to the long-form of Census 2000 due to large sampling
and non-sampling errors. The ACS sample size is too small, the
population universe is estimated, and the data collection procedures are
not consistent with Census 2000.
Thabet Zakaria
Deputy Director, Technical Services
Delaware Valley Regional Planning Commission
190 N. Independence Mall West
Philadelphia, PA 19106
Phone: 215-238-2885
Email: tzakaria(a)dvrpc.org
Fax: 215-592-9125
Here's a link to the 9/6/2006 hearing before the US House Committee on Government Reform, Subcommittee on Federalism and the Census:
2+2 Should Never Equal 3: Getting Intercensal Population Estimates Right the First Time.
Link to testimony: http://reform.house.gov/FC/Hearings/EventSingle.aspx?EventID=49552
From: Srinivasan, Nanda <FHWA>
Sent: Fri 9/1/2006 10:03 AM
To: ctpp-news(a)chrispy.net
Subject: FW: [CTPP] ACS Data on Commuting
From: Srinivasan, Nanda <FHWA>
Sent: Fri 9/1/2006 12:41 AM
To: Chuck Purvis
Cc: APICKARD(a)hrpdc.org; ctpp-news(a)chrispy.net
Subject: RE: [CTPP] ACS Data on Commuting
Andy and Chuck:
I am developing a national county based mode for workers in household for every county in the country (Table 1-35 in CTPP 2000), and will post this soon. Nationally, there were 1.339 million GQ workers in 2000, and almost half of them walked or biked (commute time (tt) <= 14 mins). National mode share (percent) with or without GQ was not that different for 2000.
Nationally, GQ travel time was around 14 minutes in 2000 (using 1 % PUMS). Nationally, GQ does not matter much and gets masked, but not at a regional level. You could use the 5% PUMS to get a feel for it for both SF and for VA beach. For the entire state of VA, GQ travel time was 13.5 minutes in 2000 (using 5 % PUMS).
Thanks
Nanda
________________________________
From: ctpp-news-bounces(a)chrispy.net on behalf of Chuck Purvis
Sent: Thu 8/31/2006 7:41 PM
To: CTPP-News; Andrew PICKARD
Subject: Re: [CTPP] ACS Data on Commuting
Andy:
I'll respond to CTPP-News since this is a good question and a tough
answer.
Yes, you can derive Group Quarters workers by Means of transportation
by subtracting TOTAL workers by means, say, taking CTPP Table 1-20
(workers by means of transportation (11) by length of US residence) LESS
CTPP Table 1-35 (workers in HHs by means of transportation to work (11)
by vehicles available in HH). This should work, UNLESS you need detailed
transit-sub-modes, or bike vs walk splits.
No, you can't get average travel time, or GQ workers by travel time
distribution (or even HH workers by travel time distribution) from the
CTPP. The only method I'm aware of to obtain AVERAGE TRAVEL TIME for
HOUSEHOLD WORKERS is from Census 2000 PUMS. Of course, with PUMS, you
can estimate the average travel time, and the travel time distributions,
for GQ workers as well as household workers. For our region, there is no
difference in the average travel time for total workers compared to
household workers, so for Average Travel Time it may not be worth the
effort to do the calculation in PUMS. It's probably region-specific: how
different are your GQ workers compared to HH workers?
We would be in big trouble with our non-motorized lobby if we didn't
separately report bicycle from walk commuters. So, what I did was use
the bike/walk split for TOTAL commuters (say, CTPP Table 1-2, by 18
means of transportaiton), and applied the bike/walk split to the
Household Workers shown in CTPP Table 1-35. Or, you could always use
PUMS, but it's easier just to use the bike/walk split from CTPP Table
1-2.
One thing you may want to try is to "age" the Census 2000 GQ data up to
2005, using the GQ population estimates that are available, at a
county-level, from the FSCPE web site. It's an educated WAG, but it
might be useful when you want to make comparisons to independent data,
like the employed resident data from the BLS' LAUS estimates.
Chuck
**************************************************************
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/
**************************************************************
>>> "Andrew PICKARD" <APICKARD(a)hrpdc.org> 08/31/06 1:57 PM >>>
Very helpful post. Had a question for you. Do you know if there is a
way to find out the number of group quarters workers, and their travel
time and mode from 2000 Census? Just trying to get an idea of who we
are missing in the 2005 ACS. Thanks for any assistance.
Andy Pickard, P.E.
Senior Transportation Engineer
Hampton Roads Planning District Commission
723 Woodlake Drive
Chesapeake, VA. 23320
Phone: 757.420.8300 Fax: 757.523.4881
apickard(a)hrpdc.orgwww.hrpdc.org
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