Yet the idea would be picked up again in the 1950s by an unlikely group: economists. During World War II, economists had begun to use gross domestic product (GDP), a measure of economic output, as a way to keep the economy afloat. GDP is a simple formula that adds together private and public expenditure, investment and trade. Fresh out of the war and in a new economic system forged at Bretton Woods, economists became convinced that the way forward depended on formulas, algorithms and models.