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Yale economist Ray Fair is one of the pioneers of modern-day election forecasting (Fair, 1978). His presidential vote equation, which was first used to predict the 1980 election, is still applied today, although, not surprisingly, it underwent several revisions in the nearly 40 years since its first publication. Basically, the model is built on four major assumptions or theories of voting:

  1. Incumbent presidents running have an advantage.
  2. Voters like change. Therefore, parties in office for two or more consecutive terms have a disadvantage.
  3. There is a slight but persistent bias favoring the Republican Party.
  4. The state of the economy affects the incumbent party vote.

Presidential vote equation

The presidential vote equation predicts the Democratic two-party popular vote. It is formulated as (Fair, 2009; 2018):

V = 48.06 + 0.673 (G*I) – 0.721 (P*I) + 0.792 (Z*I) + 2.25 (DPER) – 3.76 (DUR) + 0.21 (I) + 3.25 (WAR)

Table 1: Overview of variables used in the Fair model
VariableDescription
GGrowth rate of real per capita GDP in the first three quarters of the on-term election year (annual rate)
Pabsolute value of the growth rate of the GDP deflator in the first 15 quarters of the administration (annual rate) except for 1920, 1944, and 1948, where the values are zero.
ZNumber of quarters in the first 15 quarters of the administration in which the growth rate of real per capita GDP is greater than 3.2 percent at an annual rate except for 1920, 1944, and 1948, where the values are zero
I1 if there is a Democratic presidential incumbent at the time of the election and -1 if there is a Republican presidential incumbent
DPER1 if a Democratic presidential incumbent is running again, -1 if a Republican presidential incumbent is running again, and 0 otherwise
DUR0 if either party has been in the White House for one term, 1 [-1] if the Democratic [Republican] party has been in the White House for two consecutive terms, 1.25 [-1.25] if the Democratic [Republican] party has been in the White House for three consecutive terms, 1.50 [-1.50] if the Democratic [Republican] party has been in the White House for four consecutive terms, and so on
WAR1 for the elections of 1918, 1920, 1942, 1944, 1946, and 1948, and 0 otherwise
VDemocratic share of the two-party presidential vote

Computer your own 2020 presidential vote forecast

Entering the variable values from Table 1 for the 2020 election, the vote equation can be written as:

V = 45.6 – 0.673 (G) + 0.721 (P) – 0.792 (Z)

In the following vote equation, you can compute your own forecast of the Democratic two-party vote by modifying the values for GP, and Z in the green highlighted textboxes:

V = 45.6 – 0.673 * + 0.721 * – 0.792 *

=

The chart below shows how the forecast from the Fair model has changed since January 1st, 2016.

References

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