Why should I care if turnout rates are calculated as percentage of VAP or VEP?
There are two primary reasons why the voting-eligible population (VEP) is the preferred turnout rate denominator over the voting-age population (VAP).
The most valid turnout rates over time and across states are calculated using voting-eligible population.
Declining turnout rates, post-1971, are entirely explained by the increase in the ineligible population. In 1972, the non-citizen population of the United States was less than 2 percent of VAP and in 2004 it was nearly 8.5 percent of VAP. The percent of non-felons among the VAP have increased from .5 to about 1 percent of the VAP since the mid-1980s.
Using VEP turnout rates, recent presidential elections have returned to their levels during the high participation period in the 1950s and 1960s.
State turnout rates are not comparable using VAP since the ineligible population is not uniformly distributed across the United States. For example, nearly 20 percent of California's voting-age population is ineligible to vote because they are felons or are not citizens.
There is a use for VAP turnout rates, which is why I provide them alongside VEP turnout rates.
Some polling firms' survey universes are the voting-age population. These polling firms may wish to weight their surveys to their estimates of what the VAP turnout rate will be in an upcoming election, based on previous elections.