Obama’s approval rating averaged about 43% in polls conducted in late October. But what if it were a little higher or little lower? See how changes to it affects the model.

Use forecasted rather than known G.D.P.

Rather than assuming we know exactly what GDP will be next year, this treats the value you pick as a forecast which could turn out to be too high or too low. This will introduce more uncertainty into the presidential race forecast, but is a more realistic assumption since GDP growth is hard to predict exactly.