For example, if the Bush National exit poll deviation was
3.50% in going from from 47.0% in the preliminary poll to
50.5% in the recorded vote, then the odds are 1 in 288 billion
for this result to have occured by chance alone.
The 1.0% MOE stated by Edison/Mitofsky in the notes at the
bottom of the exit poll summary screen (WP/NEP) is used for
the calculation.
If the MOE was 1.50%, the odds are much higher that the
deviation could have been due to chance: 1 in 416,000.
We use the Normal Distribution Function, calculated over a
matrix of the two variables: Deviation and MoE (1.96* standard
deviation at the 95% confidence level). The standard deviation
= MoE/1.96 is entered in the Excel function.
The probability P is given by:
P = 1 - NORMDIST(Actual vote, Exit poll, MoE/1.96,TRUE)
P = 1 - NORMDIST(.505, .470, .01/1.96, true) = 1/288 billion.
Probability Matrix
Deviation from preliminary exit poll to the final vote
Exit Poll Margin of Error
Dev. 1.00% 1.50% 1.75% 2.00% 2.25% 2.50% 2.75% 3.00%
1 in
1.00% 40 10 8 6 5 5 4 4
1.50% 609 40 22 14 10 8 7 6
2.00% 23k 223 80 40 25 17 13 10
2.50% 2.1mm 1.8k 391 140 68 40 27 20
3.00% 485mm 23k 2.6k 609 223 107 62 40
3.50% 288bn 416k 23k 3.3k 871 330 159 90
4.00% 450tr 11.5m 267k 23k 4k 1.1k 459 223