"NCVS [National Crime Victimization Survey] estimates based on new methods Violent and property crime rates in urban and suburban areas of the United States remained stable between 2005 and 2006, according to findings from the National Crime Victimization Survey (NCVS). Due to changes in survey methodology in 2006 that mainly affected rural areas, national-level estimates were not comparable to estimates based on NCVS data from previous years. Continuity between urban and suburban areas in the sample for both years enabled year-to-year comparisons for these areas. [...] The U.S. Census Bureau, the Bureau of Justice Statistics (BJS), and a panel of outside experts extensively reviewed the 2006 NCVS data and determined that there was a break in series between 2006 and previous years that prevented annual comparison of criminal victimization at the national level. This was mainly the result of three major changes in the survey methodology  introducing a new sample to account for shifts in population and location of households that occur over time  incorporating responses from households that were in the survey for the first time  using computer-assisted personal interviewing (CAPI)."
Bureau of Justice Statistics: NCJ 219413
U.S. Department of Justice, Bureau of Justice Statistics: http://www.ncjrs.gov/