A frequently cited-source for turnout and voter registration rates is the Current Population Survey's biannual reports on voting and registration (aka CPS). The CPS is a large survey conducted by the federal government with the primary purpose of computing the unemployment rate. In the November of a federal election year, a limited supplement of voting and registration questions are asked. These data are among the best information for turnout and
registration among various demographic subgroups -- such as the
young or by race -- which cannot be computed from aggregate data presented
However, there are notable distinctions between the CPS turnout rate and the VEP turnout rate.
Second, the CPS is a survey, and is therefore susceptible to all survey methodology issues -- both from random sampling and non-random sampling issues. (Indeed, myself and others have questioned the dubious CPS practice of counting all persons who are never asked the voting question as having not voted.) Thus, even if the CPS and VEP had effectively the same denominator, these survey issues would still produce different estimates.