Bureau might consider perturbing the month of data collection or taking other steps to protect confidentiality. Similarly, the Census Bureau should consider developing selected summary tables that identify the season of collection (such as winter and summer) for geographic areas for which such information would be useful.

Recommendation 4-9: The Census Bureau should undertake research to develop confidentiality protection rules and procedures for tabulations from the ACS that recognize the protection afforded to respondents by pooling the data over many months. Whenever possible, the Census Bureau should prefer confidentiality protection procedures that preserve the ability to aggregate smaller geographic areas into larger, user-defined areas.

Collapsing Tables for Large Sampling Errors

In addition to procedures to protect confidentiality, the Census Bureau applies collapsing (or suppression) rules to the ACS 1-year and 3-year period standard tabulations that are designed to reduce the dimensions of tables, or to eliminate whole tables, that do not meet minimum standards for precision of the estimates. These collapsing rules are not applied to the 5-year period tabulations, even though the estimates will be very imprecise for small areas, because the small areas are intended to be building blocks for larger, user-defined areas.

The rules for determining which tables, or categories of tables, need to be suppressed involve examining the standard errors of every cell of a tabulation for individual tabulation areas (U.S. Census Bureau, 2006:13-10 to 13-11). For a specified table and area, the coefficient of variation (CV, the standard error of an estimate as a percentage of the estimate—see Box 2-5) is calculated for each cell of the table. If the cell entry is zero, the CV is set to 100 percent. The CV values are arrayed from high to low, and if the median CV value—the value that divides the distribution into equal halves—is greater than 61 percent, then the full table cannot be released. The categories of the table are then combined into fewer categories, and the median CV for the new table is calculated anew and the test is reapplied. If the median CV is still greater than 61 percent, then even the simpler table cannot be released (see Box 4-4 for an example).

It is difficult to evaluate this rule, but it could to lead to anomalous situations that make the data harder to use. For example, a table could be completely or partially suppressed one year and not the next year for the same geographic area, or a table could be suppressed for some, but not all, of the component areas of a large city or county. The suppression will affect small areas and minority population groups disproportionately.

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