One major reason that out-of-date registrations are not always flagged is that less than half the states participate in ERIC, a cooperative that was created after the 2012 Pew study to help make voter rolls more accurate and comprehensive. Members of the group, which currently includes 20 states and the District of Columbia, are required to share their voter registration data every 60 days. The nonprofit group uses that data - along with information from state motor vehicle departments, the Social Security death index and the U.S. Postal Service's national change of address list - to match and update voter files. In 2016, it identified about 2 million voters who had moved, passed away or had duplicate registrations.