What is the census?
Every 10 years, the U.S. government is required to conduct an "actual enumeration" of the population every 10 years. So, they conduct the census every ten years. Beginning in 1790, it has been done every decade, with the last census being in 2010.
After the 1990 census was conducted, the census bureau estimated that 4.7 million people were not counted. In addition, they found that minority groups were disproportionately undercounted, because they were more suspicious of government and less willing to cooperate with census workers. In order to correct for an undercount in the 2000 census, the Clinton administration approved a plan to estimate the characteristics of those people who failed to respond to census forms.
Department of Commerce v. US House of Representatives
In this court case, the Supreme Court ruled that sampling couldn't be used to determine the number of congressional districts each state is entitled to. However, it could be used for other purposes, such as the allocation of federal grants to states. In the end, the Bush administration decided not to use the estimation option.