People who turn to the Census Bureau’s latest data release in an effort to answer Sesame Street’s musical query may, in some cases, be puzzled by what they find. The detailed race, ethnicity and population counts make it easy to look up data for any block in America. But those numbers may not be completely accurate—and deliberately so.
A census block is the smallest unit of geography for which data are published, and blocks are the basis for assembling larger geographic entities such as legislative districts. Nationally, there are more than 11 million of them, housing on average 100 people. According to a Census Bureau description, blocks normally are bounded by streets, other prominent physical features or the boundaries of geographic areas. They may be as small as a city block or as sprawling as a 100-square-mile rural area.
The 2010 Census data being released on a state-by-state basis this month and next month, which will be used as the basis for redistricting, include counts down to the block level. Data for each block include counts not just of people in the six basic race groups, but also of people who checked any one of the dozens of multi-race combinations. The data also include counts for Hispanics and non-Hispanics in these dozens of race groups.
But what if there are only one or two people on a block who are in a different race or ethnic category from that of the other residents? In such a case, publication of this level of detail about every block in America runs the risk that a person or household could be identified individually, conflicting with the Census Bureau’s legal obligation to protect the privacy of respondents.
So the Census Bureau deliberately blurs some of its data. The total population count is correct for each block, but when it judges that there is a risk that a household could be individually identified, the bureau undertakes what is called data swapping—essentially exchanging one household in a block for another similar household nearby.
Source: Pew Research Center