So how many people are we including, or excluding, when we decide to look at the views of only registered or likely voters? In the U.S., roughly six-in-ten adults are registered to vote. The registration rate tends to tick up a few points in presidential election years (to about 65%) and back down a few points in non-election years. This means that RV polls, by design, exclude nearly 40% of adults living in the U.S.
LV polls, of course, exclude even more, as they aim to include only those registered voters who actually cast, or will cast, a ballot. The voting rate among adults ages 18 and older was 55% in the 2016 presidential election and 33% in the most recent midterm election. If a pollster is currently conducting an LV poll with 2018 (a midterm election year) in mind, then recent midterm voting behavior suggests that their results might be excluding the views of a majority of adults in the U.S.
Which population is appropriate – all adults, registered voters or likely voters – depends on what information the poll is intended to gather. For political operatives whose jobs (winning elections) require them to be in perpetual campaign mode, polls of registered or likely voters arguably provide the most accurate information about the mood and opinions of that subset of Americans who will actually cast a ballot in the next election. The fact that the poll excludes a large share of the population is a desired feature, not a bug.