RE: [CTPP] Bicycle data in ACS Journey to Work tables

Using ACS for small geographic units is difficult for means of transportation to work with very small shares, like bike. My recommendation is to use the ACS PUMS data, and determine the PUMA that best fits the area you are interested in. Each PUMA requires a minimum of 100,000 population (currently from Census 2000, as the new PUMAs are not yet defined). BUt, keep in mind that because of the small sample and the small bike shares that there will be a high margin of error in the results. Use the Univ of MN IPUMS to create your own tabulations, on line. I wrote an article in a CTPP status report April 2009 http://www.fhwa.dot.gov/ctpp/sr0409.htm, and our webinar on this topic is archived at http://ctpp.transportation.org. Elaine FHWA Office of Planning -----Original Message----- From: ctpp-news-bounces(a)chrispy.net on behalf of Ed Christopher Sent: Tue 9/21/2010 3:55 PM To: ctpp-news(a)chrispy.net Subject: Re: [CTPP] Bicycle data in ACS Journey to Work tables Page 12 of this Census Bureau Compass Report gives some general information on collapsing for statistical reasons. http://www.census.gov/acs/www/Downloads/handbooks/ACSBusinessHandbook.pdf However, little is out there on how the process actually works. Starting on slide 3 of this 2006-7 power point are some details on the mechanics of the process. If you look at these slides ignore slide 2 as the disclosure rules have changed slightly. http://download.ctpp.transportation.org/training/ACS%20data%20issues.ppt Finally If you go to NCHRP Report 588 it says this about what I call statistical collapsing... "Table Variable Collapsing: As for the 2000 CTPP, the Census Bureau expects to apply quality control measures on ACS data products for geographic areas for which categorized tables could be misinterpreted. Any table whose median distribution of covariance for individual cell values is greater than 61 percent will be modified or suppressed for that geography. For example, for a given geography for County X, for a table with 18 means of transportation, individual covariances are calculated for estimates of workers who drove alone, carpooled, etc. If the median of these covariances isgreater than 61 percent, then these modes would be collapsed to fewer categories according to predefined collapsed table definitions. If the median of covariances for the collapsed table still exceeds 61 percent, the table will be suppressed for County X." http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_588.pdf Michael Moan wrote: -- Ed Christopher 708-283-3534 (V) 708-574-8131 (cell) FHWA RC-TST-PLN 4749 Lincoln Mall Drive, Suite 600 Matteson, IL 60443 _______________________________________________ ctpp-news mailing list ctpp-news(a)chrispy.net http://www.chrispy.net/mailman/listinfo/ctpp-news