Which of the following concepts of terms of trade was formulated by G.S. Dorrance ?

"Income terms of trade" (measuring a country's capacity to import) was formulated by G. S. Dorrance. It equals the net barter terms of trade multiplied by the export volume index ($I = \frac{P_x}{P_m} \cdot Q_x$), showing the volume of imports obtainable from total export earnings.