Results current as of Tuesday, September 14, 2004 4:00PM (click here for trends over time)

Because of overwhelming interest and the start of term, I am trying out a simplified rule that I hope will save time: daily updates on electoral-vote.com as the primary source, with supplemental data from RealClearPolitics and Zogby/WSJ (subscription). Where a disagreement occurs, precedence goes to electoral-vote.com, which has rapidly become a standard resource (although its bottom-line estimate fluctuates quite a bit). Also, I could use some coding/update help, especially from the obsessive and MATLAB-savvy. Contact me.

We now have a full measure of the post-GOP convention bounce: about 10 electoral votes. This is about one-third the size of the Democratic convention bounce. The GOP bounce seems to have leveled out, but knowing for sure will take a few more days. Kerry's lead in the Electoral College is extremely slim, and currently depends on winning one of the <50% states. Note the mismatch between the electoral calculation and Bush's lead in national polls. Evidently, it is indeed possible for battleground states to diverge from overall national opinion. Statistically, this is a tie.

Today's median (expected) outcome: Kerry 273 EV, Bush 265 EV (map)

Kerry 95% confidence band: 242-309 EV

Kerry Electoral College win likelihood: 59%

The potential effects of poll bias, differential turnout, or shift in opinion are as follows. Probabilities greater than 99.99% are given as 100%.

3 points to Kerry: Kerry 327 EV (95% confidence 290-360 EV), win 99.97%.
2 points to Kerry: Kerry 310 EV (95% confidence 278-345 EV), win 99%.
1 point to Kerry: Kerry 289 EV (95% confidence 263-327 EV), win 91%.

1 point to Bush: Kerry 254 EV (95% confidence 226-284 EV), win 16%.
2 points to Bush: Kerry 233 EV (95% confidence 212-264 EV), win 1%.
3 points to Bush: Kerry 222 EV (95% confidence 205-244 EV), win 0.02%.

State-by-State Probabilities

Current probabilities of a Kerry win in each battleground state, computed from the last three polls. States in boldface have a new poll posted since Friday. Click on a state to view a tabulation of most of the polls. Some of the others come from these data sources (some are subscription-only). All data are visible in this MATLAB script.

Rank order of the battleground states. States currently in play in the 20-80% probability range are in bold.

This is a history of the calculation. For this calculation each poll is assigned to the last date on which polling was done. The marked events are inspired by a similar graph by electoral-vote.com. In my graph, the effect of events is clearer because I use polling margins and because I average over three polls. Fahrenheit 9/11, adding John Edwards to the ticket, and the Democratic convention seemed to have measurable effects within a few days. The passing of Ronald Reagan and the assault on Kerry's war heroism did not. I will update this weekly. The last update was September 11.

History of meta-analysis over time

Comments from the author (old comments)

September 9, 2004

The median electoral vote count for Kerry has dropped below 270 for the first time since June. The key event here was the probability of a Pennsylvania win dropping below 50%. Is the Bush post-convention bounce complete? It's hard to say, but considering what a radical effect comes from a 1-point swing in either direction, whenever the probability is in the 5%-95% range this election is about turnout. This is the situation in which your efforts mean the most!

September 8, 2004

There are new polls available in nearly every battleground state (see code). Note that bounces from the selection of Edwards and from the Democratic convention took 2-3 polling cycles to reach their maximum.

August 30, 2004

Now that many states are in the midrange, it may be time to focus on voter turnout operations. How important this is depends on (a) how accurate state polls are and (b) where things are headed. For what it's worth, I think that one should add a bias of +2% towards Kerry because of Democratic turnout/get-out-the-vote/motivation and because undecided voters tend to break for the challenger. Where things are headed is a question I can't answer.

August 23, 2004

You can use the bias calculation to estimate where things are headed. If you think turnout will boost your candidate by N points, add that. If you think that one candidate will gain X points at the expense of the other, add 2*X. For instance, if turnout will increase Kerry's vote by 2 points, but Bush will pick up 1.5% of voters from Kerry, then the bias is 2 - (1.5 * 2) = -1%, or 1% to Bush.

August 20, 2004

For margin of error junkies: Rachel Findley pointed out this comparison of polls to election outcomes, which finds that only 84% of election outcomes fall within the reported 95% confidence interval. This discrepancy allows a way to estimate polling errors that go beyond sampling error. If the additional error is normally distributed, an appropriate correction would be to increase the reported margin of error by a factor of 1.4.

However, this correction does not apply to my calculation because instead of relying on reported MoE, I use inter-poll data to make an independent estimate of variance.

Methods

These calculations are based on state polls from many polling organizations (data sources). The data are fed through a MATLAB script to mathematically compute all of the above results.

The first step is to calculate the probability of winning a state, taking into account the variability of polls. This is done by calculating simple statistics on the polls: average and standard error of the mean (SEM). This is then converted to a probability of a win using the normal distribution (bell-shaped curve).

The second step of the calculation is complex: it calculates the probability of every possible outcome. For 17 states the total number of possibilities is 2^17 = 131,072. Adding Colorado, Tennessee, North Carolina and Virginia makes over 2 million possibilities. In order to reduce computing time, probabilities less than 0.1% or greater than 99.9% are classified as certain outcomes. Each possibility corresponds to a different number of electoral votes (EV).

Those are then tabulated to come up with a 50th-percentile (expected) outcome, as well as a 95-percent confidence interval. The 95-percent confidence interval is particularly useful because, like the famous Margin of Error (MoE), it gives the range of outcomes that would occur 95 percent of the time based on the available information. Note that this confidence interval is very similar to the 50th-percentile outcome from a 1-point bias towards Bush or towards Kerry.

Although this calculation takes into account the variability of polls, it is important to note what it does not do. It is integrated over the last three polls (mostly 1-4 weeks), so fast swings do not show up. It does not reject any polls, nor does it account for potential bias or predict future opinion shift in any way.

Poll selection

Polls are unfiltered and equally weighted, in part because selecting data leads to unintended biases. Therefore even though some polling organizations give demonstrable and consistent outlier results, such as state-level Fox News polls (example), Fairleigh Dickinson Public Mind, and the Badger Poll, all polls are still included. I have also included Zogby Interactive polls, which have relatively untested methods but do not show measurable bias in either direction from the average.

However, even when all of the above polls are excluded the result is virtually identical. Thus the method can be tailored but is also robust enough to give a reasonable answer even with no selection of data. For a more full discussion of my methods, see this DailyKos thread.

Bias

These calculations would be affected if there is an overall poll bias, which can have a large effect in a close race. Bias could happen if polling methods do not accurately sample actual voting patterns. However, in 2000, Ryan Lizza at The New Republic compiled state polls. On the day before the election, that compilation indicated that the outcome would hinge on Florida. This matches what happened, arguing against major built-in biases in state polls.

Other factors may have an effect of unknown size, such as increased motivation by Democrats or the possibility that undecided voters will break against the incumbent.

Predicting the future

You can use the bias calculation to estimate where things are headed. If you think turnout efforts will boost your candidate by N points, add that. If you think that one candidate will gain X points at the expense of the other, add 2*X.

For instance, if you predict that turnout will increase Kerry's vote by 2 points, but Bush will pick up 1.5% of voters from Kerry, then the bias to use is 2 - (1.5 * 2) = -1%, or 1% to Bush.

History

I originally did this calculation to help think about how to allocate my campaign contributions. I believe that one can make the biggest difference by donating at the margin, where probabilities for success are 20-80%. To read a discussion click here.

In addition to Kerry's race, the Senate is within reach, I recommend that Democrats give to the DSCC and to the Senate campaigns of Ken Salazar (D-CO), Betty Castor (D-FL), Brad Carson (D-OK), and Chris John (D-LA) (Inez Tenenbaum/SC removed). Note: despite the current dip, I will leave this up for brighter days ahead. Especially give to Betty and Ken, who are from swing states!

For those of you wanting to reinforce the national election (to see why, go to the bias analysis above), I recommend the voter registration and turnout organization America Coming Together. For the optimists there is also the DCCC.

To point out the obvious, the converse interpretation of these calculations for Republicans is to direct resources toward the White House and the Senate.

Other election resources