Math Model Predicts a Bush Win

Aug. 29, 2004 -- A well-known Yale economist has written a book using the mathematical technique of regression to predict the outcome of presidential elections. Ray C. Fair's Predicting Presidential Elections and Other Things grew out of his 1978 paper that provides quite accurate descriptions of these quadrennial elections dating back to 1916. Before getting to his very surprising prediction for this November (with which I disagree), let me sketch the idea behind the technique.

To see how an adult son's height, for example, relates to his father's height, first find a random collection of father-son pairs. For each pair, measure the height of the father and the son and plot them, respectively, on the horizontal and vertical axes of a graph so that each father-son pair gives rise to a point on the graph. Examine the cloud of points thus generated and determine the closest-fitting straight line through them. Most of the points will probably be clustered near this line, known in statistics as the regression line, and its equation allows us to predict, with a certain margin of error, a son's height from that of his father.

Of course, the details and assumptions required to do this are a bit complicated, especially when the quantity in question depends on more than one factor.

In Professor Fair's model the quantity being predicted (the share of the presidential popular vote going to the incumbent party) depends on six factors. The first is which candidate is an incumbent, incumbency being a distinct advantage historically. The second is party, Republicans having a slight historical edge, and the third is "party fatigue," not being in power for more than two terms giving some benefit. The remaining three factors concern the economy: the per capita growth rate for the GDP (higher is better for the incumbent), the number of quarters during the previous 3¾ years in which the growth rate exceeded 3.2 percent (higher is better), and the inflation rate (lower is better).

If we plot these six factors along with the percentage of the two-party vote going to the incumbent political party on a graph (an impossible-to-visualize 7-dimensional graph) for each of the elections since 1916, we get a cloud of points.