Colorado (Figure 1) has shifted to undecided since my last post based on an alpha = 0.01 criterion for deciding that a percentage difference between Obama and Romney is significant. The current results for all states are in Table 1. The predicted national vote difference stands at 3.0%.
As of today, the predicted electoral vote outcome is 294 for Obama and 206 for Romney with 38 electoral votes to be decided.
Nate Silver has an interesting discussion today on the problem of calculating the probability of Obama winning. If the sampling error in polls is random error and if over a given period of time, there is no systematic change in voter decisions, then pooling polls yields a near certain decision. If there are different biases among polling groups, these biases may tend to cancel out, but whether these biases cancel out also depends on the frequency of polls from different polling groups. This adds some uncertainty but probably not much. I have tested removing Rasmussen polls, which have a Romney lean, but the electoral vote outcome is not meaningfully changed except that Colorado is significantly for Obama. Thus, these types of biases are likely of little concern.
The fundamental problem, as Nate Silver points out, is systematic bias. Due to some commonly used methods or problems in polling (such as low response rate of people polled). This could mean systematic differences between actual voting percentages from polled percentages by up to several points. If this is the case, then Romney may well win or if the bias is in the other direction, Obama may win in a landslide. As Nate Silver points out, the current state polling data indicates near certainty of an Obama win, but his 85.1% Obama win vs. 14.9% Romney win (11/3/2012 calculations) is intended to capture the systematic bias uncertainty in the prediction. That is the 14.9% probability of a Romney win is solely based on the uncertainty of systematic bias. This estimated uncertainty is based on an analysis of previous systematic biases in polling in previous elections.
I’m not convinced, however, that this uncertainty can be estimated. The hidden assumption is that methods or problems that cause systematic biases in the past are the same or similar in relevant respects to possible systematic biases that may be occurring in polling now. Since we have no theory of systematic biases and their sources that can project into the future, it seems dubious that we can generalize from the past. The best that we can say is that if there are no systematic biases, then given the relevant polling data, the probability of Obama winning is very high or near certainty.