The polling industry has been hit hard by high-profile misfires in recent years. … But exactly why the polls err often remains a mystery. Potential sources for error abound: The initial samples could be biased, the likely-voter models may not reflect the actual electorate, or voters could make last-minute decisions that make even an accurate poll wrong on Election Day. Pollsters have few tools for untangling these distinct problems.
But a new study by researchers at the Pew Research Center, pollsters themselves, offers new insights into those factors during the 2014 midterms, when polls failed to anticipate a huge Republican landslide. CONT.
Nate Cohn, New York Times