An Electoral Vote Forecast Formula: Simulation or Meta-analysis Not Required
It’s very surprising that election forecasting blogs and academics who use the latest state polls as input to their models don’t apply basic probability, statistics and simulation concepts in forecasting the electoral vote and corresponding win probabilities.
A meta-analysis or simulation is not required to calculate the expected electoral vote. Of course the individual state projections will depend on the forecasting method used. But the projection method is not the main issue here; it’s how the associated win probabilities are used to calculate the expected EV, win probability and frequency distribution.
Calculating the expected electoral vote is a three-step process:
1. Project the 2-party vote share V(i) for each state(i) as the sum of the poll share PS(i) and the undecided voter allocation UVA(i):
V (i) = PS(i) + UVA(i), i-1,51
2. Calculate the probability P(i) of winning state (i) given the margin of error (95% confidence):
P (i) = NORMDIST (V(i), 0.5, MoE/1.96, true) , i=1,51
3. Calculate the total expected electoral vote EV as the sum:
EV = ∑ P(i) * EV(i), i = 1,51
The 2004 Election Model allocated 75% of the undecided vote to Kerry and projected that he would have 337 electoral votes (99% win probability) with a 51.8% two-party vote share. The unadjusted, pristine state exit poll aggregate provided by exit pollsters Edison-Mitofsky 3 months after the election indicated that Kerry won 52.0% of the vote with an identical 337 electoral votes.
The challenger is expected to win the
majority (60-90% UVA) of the undecided vote,
depending on incumbent job performance.
After calculating the individual
state probabilities, we can calculate the EV win probability. The best, most
straightforward method is
The average electoral vote is
calculated for the 5000 election trials. Of course, the average will only be an
approximation to the theoretical value based on the summation formula. But
the Law of Large Numbers (LLN) applies: the EV average and median are usually within one or two electoral votes of the
theoretical mean. The close match between the
The 2008 Election Model includes a sensitivity (risk) analysis of five Obama undecided voter (UVA) scenario assumptions ranging from 40-80%, with 60% as the base case. This enables one to view the effects of the UVA factor variable on the expected electoral vote and win probability. Electoral vote forecasting models which do not provide a risk factor sensitivity analysis are incomplete.