Monday, November 01, 2004

Presidential Election - Final Prediction

Over the last fortnight, I have been discussing the results from a simulation model to predict the outcome of the presidential election. The moment of truth is upon us, and without further ado, let me begin with a summary of my final prediction:

  • "Best Guess" electoral college votes for Kerry => 286
  • Probability of Kerry win (>270 electoral votes) => 65.64%
Key battle-ground state victories predicted for Kerry are - FL, PA, MN, WI and IA (plus HI, ME, MI, NH, NJ, OR and WA). Safe Kerry states are - CA, CT, DE, IL, MD, MA, NY, RI, VT and DC.

Key battleground state victories predicted for Bush are - OH, NM and NV (plus AR, CO, MO and WV). Safe Bush states are - AL, AK, AZ, GA, ID, IN, KS, KT, LA, MS, MT, NC, NE, ND, OK, SC, SD, TN, TX, UT, VA, WY.

I use a simulation model to account for the uncertainty inherent in polls with small sample sizes (with ~1000 samples in most cases). Because of the statistical margin of error in all polls, the reported voter preferences are only one of many likely scenarios. Using statistical simulation methods, I can generate many such scenarios, and tally the results to predict the range of likely outcomes as well as the probability of each outcome.

Here are some of the key features / assumptions in my model.

  1. For the 18 battleground states, I use the average of polls over the last week or so as reported by Real Clear Politics. For the other states, where the lead of the candidate is beyond the sampling margin of error, I use the latest poll data from I believe using average poll numbers for the battleground states makes the analysis more robust.
  2. On a state-by-state basis, undecided voters (after considering Nader votes) are allocated between Bush and Kerry based on one of three scenarios: (a) equal spilt between Bush and Kerry, (b) favor Kerry 3:2, and (c) favor Kerry 2:1.
    • The incumbent rule suggests that undecided voters tend to break for the challenger in roughly 2:1 proportions.
    • However, this might be mitigated by the natural tendency to stay the course when national security is a prime concern.
    • On the other hand, the polls may not have correctly accounted for the preferences of first-time (especially) young voters. There is some evidence that this group tends to favor Kerry.
    • There is also the issue of increased turnout, which is likely to favor Kerry.
    • In my judgement, the net effect of all these factors will be a tip towards Kerry of undecided voters, i.e., a 2:1 bias in favor of Kerry. This is higher than what I have been using before - primarily because of the turnout factor.
  3. The likely percentage of Kerry votes is calculated by assuming it follows a normal distribution with (a) mean based on the poll results plus undecided allocation and (b) standard deviation based on the polling error.
  4. The winner for each state is allocated all of the state's electoral votes.
  5. The results are aggregated for 5000 simulations to provide the average electoral votes for Kerry and also the probability of Kerry winning more than 270 electoral votes.
  6. The "best guess" electoral vote total is obtained by adding the state-by-state totals calculated from the mean estimate of voter preferences + undecided allocation. Note that this is different from the mean of the distribution of Kerry electoral votes.
Scroll down for a graph showing the projected distribution of Kerry's electoral votes for the scenario where undecided voters favor Kerry 2:1. The blue bars denote the outcomes corresponding to a Kerry victory. As noted earlier, I am going with this scenario as my final "bottom-line" prediction.

En passant, thanks to Andrea Moro and Sam Wang for generating the media's interest in probabilistic predictive models for this Presidential Election.

Tomorrow, if all goes well, we shall be able to determine the predictive accuracy of this and other models of the presidential election. Stay tuned for the post-mortem.

Until then, 10-4.

Distribution of Kerry electoral votes - Final Prediction Posted by Hello