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Disclosure and Utility of Census Journey-to-Work Flow Data from the American Community Survey
Is There a Right Balance?
by
Ed Christopher
FHWA Resource Center
Nandu Srinivasan
Cambridge Systematics Inc.
This paper was developed to augment the display poster prepared for the Conference on Census Data for Transportation Planning: Preparing for the Future. The opinions and views expressed in this document and subsequent poster represent those of the authors (and those who have influenced them) but should not be considered the views, policy positions or in no way be attributed to the organizations for which they work or have any affiliations.
Irvine, California
May 11 to 13, 2005
Abstract
Early in 2003 the transportation community contracted with the Census Bureau to produce the CTPP2000, a special tabulation. A special tabulation is made up of user defined tables and falls outside the "standard" products distributed by the Census Bureau like SF1, SF3, and PUMS. With the 2000 decennial data, the Census Bureau required all special tabulations to have disclosure avoidance techniques applied to them. For CTPP2000 this meant the institution of rounding and threshold techniques in addition to the already applied procedures of data swapping and imputation.
The specific disclosure rules for the American Community Survey after 5 years of data collection are likely to be similar, if not stricter than to those used for CTPP2000. In this paper the effects of rounding and thresholds on the CTPP will be exposed along with an examination of their effects under the American Community Survey. CTPP2000, ACS, 1990 CTPP and the NCHRP 8-48 data sets are used in this analysis.
We show how the rounding rules cause an undercount in the published datasets. The rounding rules for CTPP2000 could have worked better had the underlying data been more closely examined for the frequency of occurrence of cell values before the rounding decision was made. Finally, we show that a minor tweaking of the rules could have produced a more consistent dataset.
As for thresholds, they will always cause severe data loss even at a medium level of geographic aggregation, let alone for small geography. Compounding the severe data loss, consider that the number of observations in a 5 year accumulated ACS will be at least 25 percent smaller than those collected from the decennial census.
The purpose of this page is to provide Bay Area data users with easy access to CTPP data and documentation. The CTPP is a special product produced by the U.S. Bureau of the Census for the transportation community. More information on the CTPP is available at the links provided at the bottom of this page.
The CTPP comprises three elements: Part 1 provides tabular data by area of residence; Part 2 provides data by area of work; and Part 3 provides commuter "flow" data from area of residence to area of work. Data is available at various geographic "summary levels" including county, places (above 2,500 population), census tracts (1,405 in Bay Area), block groups (4,422 in Bay Area), census travel analysis zones (4,069 in Bay Area), and 5% PUMAs (54 in Bay Area).
The CTPP Part 1 data for California was received July 9th, 2003.
The CTPP Part 2 data for California was received January 11, 2004. The data CD that was received on 1/11/04 contains only ASCII data and SAS jobs to analyze the datasets.
The CTPP Part 3 dataset for California was released May 6, 2004. Part 3 data contains the county-to-county, place-to-place, tract-to-tract, etc., data.
Our intent is to provide CTPP data in common data formats. We are providing data in "csv" (comma-separated value) formats for importing into spreadsheet and database applications, and we are providing data in ESRI "shp" file format for use in ArcView or ArcGIS software. Note that some of the "dbf" files associated with the "shp" files are very large with several thousand variables, and cannot be used in MS-Excel.
Americans' average commute to work got shorter by 0.4 minutes from 2000 to 2005. Here are the 25 metropolitan areas with the longest commutes to work. Commutes are in average minutes, one way:
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