Producing Transportation Data Products from the American Community Survey that Comply with Disclosure Rules: NCHRP Web-Only Document 180

摘要

The Census Transportation Planning Products (CTPP) will contain tables for about 150,000 traffic analysis zones (TAZs), other geographies, and journey to work flows, which will result in millions of tables involving dozens of variables. The main disclosure avoidance practice that had been used on certain CTPP tabulations to accomplish this objective was cell suppression. In this method, small cells were identified and suppressed, and then other related table cells that would allow the primary small cells value to be logically deduced from the tables margins also were suppressed. The small cells were defined using the Rule of 3, which certainly reduced the disclosure risk. Unfortunately, suppression would result in suppressed data in an estimated 80 percent or more of places in the nation, using a 10-level Means of Transportation (MOT) variable (Miller 2008) for three-year American Community Survey (ACS) tabulations. With the underlying data for the CTPP moving to an ACS five-year combined sample about half the size of the Census Long Form data, it is clear that the data loss due to Census Bureau Disclosure Review Board (DRB) disclosure rules at finer geographic areas would have been substantial. In fact, the results of the initial risk assessment on the national sample for the 2005-2009 ACS, which identified data values at most risk of disclosure, determined that about 90 percent of the TAZs were affected by DRB rules for at least one table, which would have triggered the cell suppression. The main goal of the project was to arrive at an operationally practical data perturbation approach that satisfies the transportation data user community's analytical needs while simultaneously satisfying the disclosure rules set by the DRB.