"A commonly asked question about naturalization is whether immigrants from certain countries are more likely to become U.S. citizens than immigrants from other countries. Two primary data sources commonly used to calculate naturalization rates are the U.S. Census Bureau's decennial census and surveys, and the administrative records of the Department of Homeland Security (DHS). Naturalization rates computed from these two data sources, however, may be quite different. This Office of Immigration Statistics Fact Sheet explains why this happens and provides illustrative examples."
DHS Office of Immigration Statistics: http://www.dhs.gov/ximgtn/statistics/
Fact Sheet (November 2006)