You know how we read articles about the results of studies and carrying headlines like "In 2020 Number of Octogenarian Turtle Farmers Will Outnumber Septuagenarian Muskrat Herders."  Many of those studies use data sub-sets of the US Census that are made publicly available by the US Census Bureau for exactly that purpose.  The problem is that those data sub-sets have some glaring errors:. 

The errors are documented in a stunningly straightforward manner. The authors compare the official census count (based on the tallying up of all Census forms) with their own calculations, based on the sub-sample released for researchers (the “public use micro sample,” available through IPUMS). If all is well, then the authors’ estimates should be very close to 100% of the official population count. But they aren’t...

These microdata have been used in literally thousands of studies and countless policy discussions. While the findings of many of these studies aren’t much affected by these problems, in some cases, important errors have been introduced. The biggest problems probably exist for research focusing on seniors. Yes, this means that many of those studies of important policy issues—retirement, social security, elder care, disability, and medicare—will need to be revisited.

It's kind of hard to make good policy decisions if they're based on inaccurate information.  Still, no one is disputing the accuracy of the census itself which is important to remember as we gear up for the 2010 count.  Hopefully The Census Bureau will be diligent in making sure that the data sub-sets that are generated from the new count are far more accurate than the 2000 versions.