HUFFINGTON POST: …The basic idea is straightforward: Gallup uses answers to survey questions to identify the adult respondents who seem most likely to vote. In practice, that means asking a series of questions about voter registration, intent to vote, past voting, interest in the campaign and knowledge of voting procedures — all characteristics that typically correlate with a greater likelihood of actually casting a ballot — and combining responses to those questions into a seven-point scale. Those respondents who score highest on the scale are classified as “likely voters,” after Gallup makes a judgment call about its cutoff point — that is, the percentage of adults that best matches the probable level of voter turnout.
Until the fall of an election year, most national pollsters choose to report their survey results for the larger population of self-described registered voters. But in the final weeks of the campaign, Gallup and others shift to the narrower segment of likely voters, which has typically made their estimates more accurate by filtering out registered voters who aren’t likely to go to the polls on Election Day…
One theory as to why the pool of self-described registered voters so closely resembled the actual electorate is that many non-likely voters were, in effect, already screening themselves out — by opting out of the survey. As the Pew Research Center reported in May 2012, actual voters are already more likely to respond to its surveys, while non-voters are more likely to hang up. “This pattern,” Pew wrote, “has led pollsters to adopt methods to correct for the possible over-representation of voters in their samples.”… (more)
