A True Vote Probability Analysis of a Kerry win in Ohio and Florida

 

Richard Charnin

Jan. 8, 2012

 

A table of win probabilities has been added to the True Vote Model (TVM). 

 

https://docs.google.com/spreadsheet/ccc?key=0AjAk1JUWDMyRdGN3WEZNTUFaR0tfOHVXTzA1VGRsdHc#gid=0

 

The TVM is a post-election model which calculates vote shares and margins based on various estimates of returning voters from the prior election. The TVM uses intermediate vote shares of returning and new voters (based on the national exit poll) to determine total vote shares. 

 

There are four calculation methods which estimate voter turnout from the prior election: 1- recorded vote, 2-votes cast (recorded+uncounted), 3- Exit Poll, 4- True Vote.

New voters are simply the difference between returning voters and total votes cast in the current election.

 

Because of the inherent uncertainty in these estimates, the TVM calculates vote shares over a range of the estimated shares of returning and new voters (scenarios). Two 5x5 tables of resulting vote shares and corresponding vote margins are generated. The most likely base case vote share in the central cell of the table, the worst case share is in the lower left cell, and the best case share is in the upper right cell. This is called a sensitivity analysis of vote share scenarios. 

 

The win probability in a pre-election or exit poll is a function of the 2-party shares and the polling margin of error, calculated by the normal distribution function.

For example, assume a 51-49% vote share split and a 3.0% input margin of error (MoE).

 

The probability of winning a majority of the vote is given by the Excel formula:

Prob = NORMDIST (.51, .50, .03/1.96, true)

Win probability = 74.3%

 

Ohio 2004 presidential election

In the base case (most likely) scenario, Kerry wins 59.5% of New voters and 10% of returning Bush 2000 voters.

The probability calculations assume a 3.0% margin of error.

 

The base case (most likely) scenario in the central cell of the table:

Kerry captures 59.5% of new voters and 10% of returning Bush 2000 voters.

Kerry wins Ohio with a 51.7% share (a 258,000 vote margin). His win probability is 94%.

 

The best case scenario in the upper right cell:

Kerry captures 63.5% of new voters and 12% of returning Bush 2000 voters.

Kerry has 53.2%, a 427,000 vote margin and 99.5% win probability.

 

The worst case scenario in the lower left cell:

Kerry captures 55.5% of new voters and 8% of returning Bush 2000 voters.

Kerry has 50.2%, a 90,000 vote margin and 70.6% win probability.

 

Florida 2004

Bush won the recorded vote 52.1-47.1%, a 381,000 vote margin.

 

Now let's calculate Kerry's probability of winning over a range of scenarios:

He wins 52.2-60.2% of new voters and 8-12% of returning Bush voters. Assume the same calculation method (returning voters are based on the 2000 exit poll).

 

Base Case: Kerry has 52.3%, 427,000 margin and 97.2% win probability.

Best Case: Kerry has 54.0%, 678,000 margin and 99.99% win probability.

Worst Case: Kerry has 50.6%, 176,000 margin and 78.5% win probability.

 

National 2004

Bush won the recorded vote: 50.7-48.3%, a 3.0 million vote margin.

 

Kerry's vote share scenarios:

Share of new voters: 53-61%. Share of returning Bush voters: 8-12%.

 

Base Case: Kerry has 52.9%, 9.0 million margin and 99.1% win probability.

Best Case: Kerry has 54.4%, 12.8 million margin and 100.0% win probability.

Worst Case: Kerry has 51.4%, 5.3 million margin and 91.9% win probability.

 

 

Base Case

Ohio                True Vote Model Method 3                                        Unadjusted Ohio Exit Poll                            

 

2000                Voted  Mix      Kerry   Bush    Other   Turnout           Mix      Kerry   Bush    Other

New/DNV      995      18.1%  59.5%  38.5%  2.0%    -                       18.1%  62.2%  35.8%  2.0%

Gore (47%)     2,109   38.4%  91.0%  8.0%    1.0%    98%                 38.4%  95.1%  3.9%    1.0%

Bush (49.5%)  2,221   40.5%  10.0%  90.0%  0.0%    98%                 40.5%  10.4%  89.6%  0.0%

Other (3.6%)   160      2.9%    64.0%  17.0%  19.0%  98%                 2.9%    66.9%  14.1%  19.0%

                                                                                                                       

            True Vote        100%   51.7%  47.0%  1.3%                            100%   54.0%  45.5%  0.5%

            Recorded        100%   48.7%  50.8%  0.5%                                                   

 

            Kerry Share of New Voters (DNV)                                      

 55.5% 57.5%  59.5%  61.5%  63.5%

Share

of Bush           Kerry Vote Share                               

12%     51.8%  52.2%  52.5%  52.9%  53.2%

11%     51.4%  51.7%  52.1%  52.5%  52.8%

10%     51.0%  51.3%  51.7%  52.1%  52.4%

9%       50.6%  50.9%  51.3%  51.7%  52.0%

8%       50.2%  50.5%  50.9%  51.3%  51.6%

 

                        Kerry Margin (000)                

12%     268      307      347      387      427

11%     223      263      303      343      382

10%     179      219      258      298      338

9%       134      174      214      254      294

8%       90        130      169      209      249

                                                           

                        Kerry Win Probability                                                                        

12%     94.7%  96.8%  98.2%  99.0%  99.5%

11%     91.1%  94.4%  96.6%  98.1%  98.9%

10%     86.0%  90.6%  94.0%  96.4%  97.9%

9%       79.1%  85.3%  90.2%  93.7%  96.2%

8%       70.6%  78.3%  84.7%  89.7%  93.4%