How to improve polling? Ask voters who will win.
A new national CNN/ORC poll made headlines Tuesday morning. This poll gives GOP nominee Donald Trump a marginal 1-point lead over Democratic nominee Hillary Clinton among likely voters, and 2 points in a four-way contest including Libertarian Party nominee Gary Johnson and Green Party nominee Jill Stein. The results are thus in stark contrast to the organization’s previous survey conducted in late July, right after the conventions, which had Clinton up by 9 points.
Major swings in polling numbers are a well-known phenomenon in presidential campaigns. The question is whether such variations reflect true changes in people’s vote intention and thus provide an accurate assessment of what is going to happen on Election Day. Research suggests that they don’t.
In comparing the results of both polls, Philip Bump at The Washington Post’s “The Fix” argues that the increasing support for Trump is due to a combination of three factors. First, Trump has consolidated support among Republicans and, second, he has improved his standing among independents. Third, ORC changed the composition of their sample: While the previous survey polled registered voters, the new survey included likely voters.
This methodological change may in fact be the most important reason for the discrepancy in the results of both polls. Recent research by Andrew Gelman at Columbia University suggests thatapparent swings in vote intention represent mostly changes in sample composition – READ MORE.