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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.
Journey To Work (JTW)
The journey to work and related questions are asked on the decennial census long form. In 1990 these data appear in Census Summary tape File #3 (STF3) and in 2000 on Census Summary File #3 (SF3). Tabulations, called the 1990 and the 2000 "Census Transportation Planning Package (CTPP)" were created for transportation analysts from these files. The CTPP has three components, a residential summary, a work place summary and a flow based summary. Most of the questions received regarding Journey to Work have been focused on the mode used and the average travel time. To this end, two interactive databases have been developed to allow users to select residence origin(s) and workplace destination(s) at the county or place level and obtain residence or workplace summaries for mode used and average travel time to work.
The 1990 application is based on the 1990 CTPP Part C, means of transportation to work table which did not contain the field “work at home.” The 2000 application is based on the 2000 CTPP Part 3 means of transportation to work table which does contain the field “work at home” whenever the origin (residence) and the destination (workplace) geography are the same. What this means is that in 1990 the workers who “work at home” need to be added to the 1990 flow data, whenever the origin and destination locations are the same, if a correct comparison is to be made with 2000.
A table exists at this location that compares 2000 with 1990 whenever the geography in 2000 matches 1990. This table contains work at home in 2000 and 1990. The reader should add the 1990 values for “workers at home” to the 1990 flow data (from the application) whenever the origin and workplace geography are the same.