2016 Postmortem
In reply to the discussion: Bernie Sanders Breaks 20% in a National Poll for the First Time; Gaining Support Across Demographics [View all]jeff47
(26,549 posts)What it means is they're adjusting the polling results based on their guess about who will actually show up to vote. That guess is based on past elections, and will trend more conservative than Democrats as a whole (Conservative Democrats vote more reliably).
One of the main way Sanders could win is by getting people who do not normally vote to actually get to the polls. Since they are effectively not being counted by "voter" polls, the results of the poll would underestimate Sanders support.
On the flip side, excitement could wane and those voters could stay home anyway, thus overestimating Sanders support. That's more or less what happened to Dean in Iowa in 2004. He had lots of excitement from people who didn't bother to show up.
This early, it is extremely difficult to create a good guess about who will actually show up at the elections. So any poll with "voters" is inherently making a pretty wild guess. Closer to the elections, it will be far easier to gauge how likely various groups are to actually vote.