Article about DRB (December 6, 2001)

The CTPP Working Group (AASHTO, Census Bureau Journey-to-Work and Migration Branch, FHWA, FTA, and BTS) has been working with the Census Bureau's Disclosure Review Board (DRB) to determine which tables can be included in the CTPP 2000.  The preliminary list of CTPP tables has been posted on the CTPP web page since October 2000. The Working Group’s Census Bureau representative, Phil Salopek, submitted the proposed list of tables to the DRB for their review. The DRB has the responsibility to ensure that individuals cannot be identified through published Census Bureau products. 

The DRB reviewed the proposed tables on August 20, 2001.  On the basis of this initial review, the DRB approved the list of residence-only and workplace-only tables with the important contingency that all cells would be rounded.  For the home-to-work tables, the DRB approved publication subject to the geographic reporting restriction that each flow combination has a minimum of 50 unweighted records before it could be released.  In effect, this restriction would eliminate nearly all TAZ-to-TAZ reporting in the CTPP, and in some cases, would even prevent county-to-county tabulations. 

In response, the Director of BTS and the Administrators' of FHWA and FTA wrote to the Acting Director of the Census Bureau and alerted him to the importance of the CTPP, the damage the DRB recommended threshold would cause and expressed hope that the Census Bureau and the transportation community could agree on a way to provide usable tables without eliminating small area tabulation.

Following from this, the CTPP Working Group met with a sub-group of the DRB to understand their rationale for the 50 unweighted records rule and to explain the importance of small area (TAZ) geographic flow data to the transportation planning community.    During the discussions, DRB representatives expressed support for the publication of a TAZ to TAZ  home-to-work flow table and answered questions from the CTPP Working Group regarding alternatives to protect individual identities.   On the basis of this discussion, the CTPP Working Group submitted to the DRB a revised set of tables that favor simple 1-way tabulations for counts, but also includes 2-way tables for summary measures (means, aggregates).  The revised list, although considerably less comprehensive than the original CTPP 2000 request, includes tables that were not available in the 1990 CTPP.    

As of November 30, the DRB has not approved the revised set of tables proposed by the CTPP Working Group.  

CTPP 2000: Revised List of Part 3 Tables for Small Geography

Rounding Values
It is the DRB’s policy that all special tabulations (such as the CTPP) will have all counts rounded.  Special tabulations are those files that are not part of the Census Bureau's "standard tabulations" such as Summary Files 1, 2, 3, and 4.  Specifically, the DRB’s rounding rules are:   "0" is "0"; values between 1-7 will be "4", and values of 8 and above will be rounded to the nearest 5 (e.g. 10, 15, 20, 25).  

The CTPP Working Group believes that many disclosure avoidance methods have already been applied to the data, so that rounding as an additional method is not necessary.   The potential disclosure of an individual is already reduced because the Census Bureau has instituted data swapping (transferring data from one individual to another individual in a "nearby" block) and  because variables such as income and place-of-work are the result of imputation and allocation in a large number of cases.  

However, in the spirit of cooperation, the CTPP Working Group is accepting the DRB rounding policy and asking that the counts of total persons, total households and total workers not be rounded and that the Census Bureau provide marginal totals rounded independently of the internal cells, and to  pursue rounding procedures that promote internal consistency of the tables within CTPP.