Sex offender registries in the United States exist at both the federal and state levels. They assemble information about persons convicted of sexual offenses for law enforcement and public notification purposes. All 50 states and the District of Columbia maintain sex offender registries that are open to the public via Web sites, although information on some offenders is visible to law enforcement only. According to NCMEC, as of 2015 there were 843,260 registered sex offenders in United States. Sex Offender Registration and Notification (SORN) has been studied for its impact on the rates of sexual recidivism for registered sex offenders, with the majority of studies demonstrating no impact.