reasonable, then having the same average yearly minimums for the 3-year and 5-year period estimates makes sense: even though the 3-year and 5-year period estimates are published for smaller geographic areas than the 1-year period estimates, they represent averages over longer periods of time.

A second rule is that 5-year period estimates of mode of transportation to work cross-tabulated by another variable will not be published for an area for a particular mode unless it has at least 3 unweighted workers in the sample. If it does not, then the mode must be collapsed with other modes to reach the minimum sample size requirement. Such a restriction is not imposed on the 1-year or 3-year period estimates. Given the skewed distribution of mode of transportation in the United States, whereby three-fourths of the population drives alone to work, another 10 percent carpools, and very small percentages take public transit, bicycle, walk, or work at home, this restriction may curtail the publication of needed information on transportation to work in many areas. In turn, such curtailment will handicap users who want to aggregate data for traffic analysis zones into larger areas of their own definition.

The reason for the restriction for 5-year period estimates is not clear. Mode of transportation to work is highly variable: the same individual may decide to walk to work in the summer and drive in the winter or may walk to work for 4 years and then decide to switch to a new bus line or vice versa. Collectively, the workers in a traffic analysis zone are unlikely to include the same individuals over the 5-year period because of changes in residence and employment.

The Census Bureau has time before 5-year period estimates become available in which to develop appropriate confidentiality protection strategies and techniques for transportation tables and other data products. Such strategies should seek to minimize disclosure risk in ways that recognize the protection afforded by averaging over 60 months of data. When developing confidentiality protection procedures for cross-tabulations, the Census Bureau should also, whenever possible, prefer procedures that make it possible to aggregate the data for smaller units into larger units. Thus, instead of suppressing cells of a cross-tabulation, it might be possible to use techniques that perturb the data for individual cells while preserving the marginal totals for each variable.

Confidentiality Protection Recommendations

Recommendation 4-8: Because of the potential value of month of data collection for analysis of the ACS PUMS, the Census Bureau should revisit its decision to omit this variable as a confidentiality protection measure. If further research determines that including exact month of data collection would significantly increase disclosure risk, the Census

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