Ray Fair, an economist at Yale, famously devised an equation linking the outcome of presidential elections to inflation and output growth, but mid-terms are murkier.

• 耶鲁大学经济学家RayFair设计了一个很有名的公式，把总统选举的结果与通货膨胀、产出增长率结合起来，但中期选举似乎更不可测。