On Predicting Voter Turnout Based On Early Voting
In July, 2012, as the runoff in the Republican and
Democratic primary election approached, Professor Mark Jones argued that voter
turnout in the Republican primary runoff election could be predicted based on
the early voting turnout in the most populous fifteen counties in Texas
(his blog post is here).
He provided early voting figures from the first Republican primary, which
allowed twelve days for early voting, compressed the data into five days to be
comparable to the five days of early voting allowed in the runoff primary
election. He stated that if a comparison of early voting (early voting in the
runoff/early voting in the first primary) was a low percentage, then turnout
would likely be closer to the low estimate for voter turnout in the runoff
primary. On the other hand, if the proportion were large, then turnout would
likely be closer to the high estimate for voter turnout in the runoff primary.
His figure is reproduced below:
Professor Jones admits that accurately predicting turnout
and the percentage of the vote that would be cast during early voting is
difficult, but he offers the following:
It is difficult to predict both what
actual turnout will be in the marquee race of the GOP primary, the Senate
runoff between David Dewhurst and Ted Cruz (in the first round in May it was
1.4 million), and what proportion of the voters will vote early or by mail (in
the 15 most populous counties, 51 percent of voters cast a ballot in the
Republican Senate primary early or by mail during the early voting period of
May 14 to May 25). A reasonable estimate of overall turnout in the GOP Senate primary
would fall between 750,000 and one million, while it is very likely that the
proportion of these Texans casting a vote early or by mail will be somewhere
between 55 percent and 60 percent.
How well did the prediction work?
Here are the data:
County
|
Rep Primary
|
Rep Runoff
|
% RR/RP
|
Harris
|
78,441
|
70,481
|
89.85%
|
Dallas
|
31,312
|
33,895
|
108.25%
|
Tarrant
|
40,463
|
34,837
|
86.10%
|
Bexar
|
36,413
|
32,764
|
89.98%
|
Travis
|
17,535
|
16,056
|
91.57%
|
Collin
|
26,581
|
22,650
|
85.21%
|
El
Paso
|
5,857
|
2,301
|
39.29%
|
Denton
|
18,774
|
14,338
|
76.37%
|
Fort Bend
|
20,884
|
14,451
|
69.20%
|
Hidalgo
|
4,293
|
2,022
|
47.10%
|
Montgomery
|
22,792
|
17,268
|
75.76%
|
Williamson
|
16,303
|
9,821
|
60.24%
|
Nueces
|
6,473
|
3,731
|
57.64%
|
Galveston
|
14,437
|
11,544
|
79.96%
|
Cameron
|
2,939
|
1,825
|
62.10%
|
Total
|
343,497
|
287,984
|
83.84%
|
EV + ED Vote
|
1,406,648
|
1,111,938
|
79.05%
|
Predicted Vote
|
1,179,333
|
||
Difference
|
67,395
|
6.06%
|
As you can observe from the table,
the turnout in the Republican runoff primary was 1.1 million voters, and the
early vote in the fifteen largest counties constituted 25.90 percent of the
total votes cast, which is greater than the 24.42 percent provided by the early
vote in the fifteen largest counties in the first primary. The total vote cast
in the runoff primary in the fifteen largest counties was nearly 84 percent of
the vote cast in the first primary. Using the early vote in the largest fifteen
counties to predict the total vote in the runoff primary resulted in a
predicted vote of 1.179 million votes, which are only 67,395 votes more than
the actual vote and a reasonably small error.
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