The Census Bureau says

"The U.S. Census Bureau collects race data in accordance with guidelines provided by the U.S. Office of Management and Budget (OMB), and these data are based on self-identification. The racial categories included in the census questionnaire generally reflect a social definition of race recognized in this country and not an attempt to define race biologically, anthropologically, or genetically. In addition, it is recognized that the categories of the race item include racial and national origin or sociocultural groups. People may choose to report more than one race to indicate their racial mixture, such as 'American Indian' and 'White.' People who identify their origin as Hispanic, Latino, or Spanish may be of any race."

http://quickfacts.census.gov/qfd/meta/long_RHI525211.htm

A similar statement is found as footnote 7 in the Census Brief 2010 "Overview of Race and Hispanic Origin: 2010"

http://www.census.gov/prod/cen2010/briefs/c2010br-02.pdf

"The race categories included in the census questionnaire generally reflect a social definition of race recognized in this country and are not an attempt to define race biologically, anthropologically, or genetically. In addition, it is recognized that the categories of the race question include race and national origin or sociocultural groups."