amounts into intervals of $10,000 or more, with a wide top category, such as $100,000 or more.
Top-coding refers to assigning a value to an individual record that is the same as that assigned to other individuals, all of whom have actual values above a specified limit. For example, all individuals with wages and salaries of $100,000 or more might be assigned the value of $100,000. Top codes for the ACS PUMS files are developed separately according to the distribution of responses by state.9
The panel strongly supports the protection of respondents’ individual information, because a breach of confidentiality would not only undercut the Census Bureau’s ability to collect information, but also break trust with respondents. At the same time, the panel is concerned that confidentiality protection not be ratcheted up without a careful consideration of the need not only to minimize disclosure risk, but also need to provide useful information for public- and private-sector decision making, research, and analysis. It is not possible to reduce the risk of disclosure to zero; the goal instead must be to minimize risks while not unduly suppressing valuable information.
Microdata Products A recent report of a panel of the Committee on National Statistics, Expanding Access to Research Data (National Research Council, 2005), addresses issues in balancing confidentiality and privacy protection with obtaining an adequate return on taxpayers’ investment through providing users with access to rich microdata sets from government surveys. The report recommends research on techniques for providing useful, innovative public-use microdata sets that increase informational utility without increasing disclosure risk.
In the context of ACS microdata, the panel encourages the Census Bureau to revisit its decision not to include month of data collection on the PUMS as a confidentiality protection measure. Given that individual PUMS records are not identified geographically for areas with fewer than 100,000 people, it could be argued that omitting month of data collection is not necessary to protect confidentiality. Including this variable on the PUMS files would be immensely valuable for analytical purposes in light of the moving ACS reference period. For instance, knowing the month of data collection would permit data users to make their own adjustments for inflation for income amounts (see Section 4-D.3 below). It would also fa-
See, for example, http://www.census.gov/acs/www/Products/PUMS/C2SS/minmaxval4.htm.