The variables: Lewis-Beck and Tien’s “Jobs model” uses five variables: whether or not an incumbent is running, the incumbent’s popularity rating according to the first July poll by Gallup, Gross National Product (GNP) growth between the second quarter of the election year and the last quarter of the previous year, growth in jobs over the first 3.5 years of a president’s term, and one for the closeness of the relationship between the incumbent president and the incumbent party’s candidate.

How well it does: The 2008 version of the model, the most recent one for which we have a full specification, had a standard error of 1.43 percent, and explained 94 percent of variation between president elections.

What it predicts: We don’t have a copy of Lewis-Beck and Tien’s latest paper, but the authors passed along their estimate that Obama will get 48.2 percent of the two-party vote share and Romney will get 51.8 percent, an estimate of which they have, coincidentally, 48.2 percent certainty.