Title:      Bayesian Combination of State Polls and Election
Forecasts

Authors:    Kari Lock, Andrew Gelman

Entrydate:  2008-09-21 16:59:28

Keywords:   election prediction, pre-election polls, Bayesian
updating, shrinkage estimation

Abstract:   In February of 2008, SurveyUSA polled 600 people in
each state and asked who they would vote for in either
head-to-head match-up: Obama vs. McCain, and Clinton vs. McCain.
Here we integrate these polls with prior information; how each
state voted in comparison to the national outcome in the 2004
election. We use Bayesian methods to merge prior and poll data,
weighting each by its respective information. The variance for
our poll data incorporates both sampling variability and
variability due to time before the election, estimated using
pre-election poll data from the 2000 and 2004 elections. The
variance for our prior data is estimated using the results of
the past nine presidential elections. The union of prior and
poll data results in a posterior distribution predicting how
each state will vote, in turn giving us posterior intervals for
both the popular and electoral vote outcomes of the 2008
presidential election. Lastly, these posterior distributions are
updated with the most recent poll data as of August, 2008.

http://polmeth.wustl.edu/retrieve.php?id=836

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