Apropos of my Q&A on early polling, Chris Wlezien sends the following, which is forthcoming in a book with Robert Erikson. They take all of the polls from up to 300 days before the presidential elections from 1956-2008 (except for 1968, which did not have polling that far back). They then forecast each presidential election outcome with the polls, starting 300 days before the election and continuing day-by-day until Election Eve. The figure below plots the trend in the r-squared from the forecasting equation. If polls were perfect predictors of the outcome, the r-squared would be 1.0. If the polls were pretty much useless predictors, the r-squared would be 0.
The graph shows that polls 300 days have little predictive value at all. The r-squared values increase sharply during the next 3 months or so — when the eventual nominees are becoming better known during the primaries — and then increase more sharply again in the 3 months before the election, when the general election campaign is underway.
I interpret this as a useful cautionary tale about over-interpreting early polls.