Social scientists have come to rely very heavily on statistics. in retrospect the crucial step was probably the development of national income accounts in the 1930s to illustrate the Keynesian model of macro-economics. By bringing out the connections between various components of demand and supply, this made it possible to estimate the effects of policy variables,(especially tax revenues and government investment) on output (and therefore employment) and on the balance of payments. Economists gained much more insight into how an industrial economy operated and this increased their usefulness to politicians and administrators. National income accounting had practical uses for demand management by governments intent on avoiding unemployment after the war, and the system of national accounts could later be adapted to quantify Harrod-Domar growth models, forming the core of most of the 'development plans' of the post-war period.