Both the Uniform Crime Reports (UCR) and the National Crime Victimization Survey (NCVS) were designed to gather and report on the United State's national crime rates. While both have their limitations and therefore inaccuracies, one difference between the UCR and NCVS is that NCVS is far more likely to be inaccurate due to its methods of collecting data.
The UCR is a cooperative effort for nation-wide law enforcement agencies to report on national crime statistics. According to the FBI's "Summary of the Uniform Crime Reporting Program," every month, "18,000 city, university and college, county, state, tribal, and federal law enforcement agencies voluntarily [report] data on crimes brought to their attention" (as cited in "Uniform Crime Reports"). The UCR first began in the 1920s and 30s. By 1930, the Federal Bureau of Investigations (FBI) was made responsible for compiling the data, though the FBI does not and has never collected the crime statistic data itself. The UCR divides crimes into two major categories called Part I offenses and Part II offenses. Part I reports data on "violent and property crimes" ("Uniform Crime Reports"). The list of crimes included under the violent crime classification are "aggravated assault, forcible rape, and robbery" ("Uniform Crime Reports"). The list of crimes classified under property crimes include "arson, burglary, larceny-theft, and motor vehicle theft" ("Uniform Crime Reports"). Part II reports on crime categories like "simple assault, curfew offenses and loitering, embezzlement, forgery and counterfeiting, disorderly conduct, driving under the influence, drug offenses, fraud, gambling, liquor offenses, offenses against the family, prostitution, public drunkenness, runaways, sex offenses, stolen property, vandalism, vagrancy, and weapons offenses" ("Uniform Crime Reports"). Since the UCR is composed of data reported monthly by law enforcement agencies, it is the most accurate crime statistics report we have; however, one of its weaknesses is that reporting crime statistics data for the UCR is not mandatory for all state law enforcement agencies. Therefore, some of the crime statistics may be "under-reported" ("Uniform Crime Reports"). A second weakness is that the crimes reported are only what the police have noted or made arrests for and not the crimes that have been sentenced at the judicial level.
In contrast to the UCR, the NCVS is merely a crime report conducted by the Bureau of Justice Statistics through surveys under the United States Census Bureau. The surveys include only 49,000 to 77,400 households two times a year. The United States Census Bureau chooses a random cluster of households to interview concerning violent crimes and continues to survey the household sample for three years, following up with more interviews every six months. The types of violent crimes that the surveys gather statics on include "assault, burglary, larceny, motor vehicle theft, rape, and robbery," as well as domestic violence ("National Crime Victimization Survey"). However, critics of the NCVS argue that the data is unreliable, especially when it comes to domestic violence, because only the "entire selected household is interviewed" rather than interviewing individual people ("National Crime Victimization Survey"). Individual people are more likely to be honest about domestic violence than heads of the household. The data is further unreliable in comparison with the URC because only a select few households are interviewed, whereas the URC attempts to be a report of all crime statistics.