CTPP September 2006

ctpp@listserv.transportation.org

8 participants
14 discussions

by ed christopher

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)

17 years, 8 months

[CTPP] (CTPP) 2005 ACS Data Release

by Zakaria

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

17 years, 8 months

[CTPP] Congressional Hearing on Intercensal Population Estimates

by Chuck Purvis

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

17 years, 8 months

FW: [CTPP] ACS Data on Commuting

by Nanda.Srinivasan＠dot.gov

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.org www.hrpdc.org _______________________________________________ ctpp-news mailing list ctpp-news(a)chrispy.net http://www.chrispy.net/mailman/listinfo/ctpp-news

17 years, 8 months

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CTPP September 2006