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Polls taken in late 1980 and 1984 varied by up to 20 points in the spread they reported between Democratic and Republican identifiers. We found three systematic and sizable differences across polls: polls that sample only voters, polls that emphasize “today” or the present in their question wording, and polls that are taken close to election day (at least in circumstances like those of 1984) tend to favor the party currently advantaged in the presidential race.
Borrelli et al. (Thu,) studied this question.