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Evaluating National Polls

October 18th, 2004, 12:00pm by Sam Wang


Although I don’t analyze national polls, I am asked about them frequently. For instance, how to interpret the latest Gallup poll reporting a Bush-Kerry margin of 8%? My brief reply: if you look carefully at all available polls, the race is closer than this single poll indicates. Consider the following.

Imagine that the race were perfectly tied and the margin of error were 4 points. In this case six measurements of the Bush-Kerry margin could easily be: Kerry +2, Bush +2, tie, Bush +6, Kerry +6, Kerry +1. Add the fact that the CNN/USA Today/Gallup poll is somewhat favorable to Bush compared with other polls, and one can see the problems with interpreting any one poll. If a national horserace summary is required for the sake of curiosity, then looking at an average (here is more data) or a median is better. If one does this, Bush is currently about 2 points up on Kerry among decided voters.

This brings me to the biggest point of all: undecided voters are not counted in point spreads, yet history suggests that most of them vote against the incumbent. This suggests that Bush’s true threshold separating victory from defeat is about 49%; he is currently slightly below that. This is the big story among pollwatchers this week. For a discussion see these L.A. Times and CNN articles.

Tags: 2004 Election

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