Some economists also use sets of statistics called diffusion indexes to calculate economic turning points. A diffusion index is a method of summarizing the common tendency of a group of statistical series. If a greater number of the series are rising than are declining, the index will be above 50; if fewer are rising than declining, it will be below 50. In effect, a diffusion index measures the degree to which either strength or weakness pervades the economy. If, for example, most of a group of industries are increasing their production rates, the economy as a whole is probably expanding; if the proportion of industries that are growing begins to decline and falls significantly below 50 percent for a period of time, the economy is probably in a recession, or at least moving in that direction.

Economists frequently use mathematical equations to express the normal relations between various economic factors. As a simple example, a given increase in consumer income will ordinarily produce a certain increase in sales, saving, and tax revenue, and these developments can be expressed mathematically. With a sufficient number of equations, all the important interactions within the economy can be simulated in a mathematical model. With the advent of computers able to make millions of calculations in a few moments, economists began to construct more and more complex sets of equations, called econometric models. These models, some of which include hundreds of equations, can be used to forecast overall economic activity (macroeconomic forecasting) or developments in particular parts of the economy (microeconomic forecasting). The success of econometric forecasting has so far been limited because the exact nature of economic relations is not fully known, and also because of the inadequacies of existing statistics. Nevertheless, the improvement of these techniques represents the greatest hope for more accurate economic forecasting in the future.

The computer has also stimulated development of another potential forecasting tool, input–output analysis. Input–output tables show the relations between the various industries and sectors of an economy. They show, for each industry, the amount of its output that goes to every other industry to be used as raw materials or semifinished products, as well as the amount that goes to the final markets of the economy. Input–output tables also show each industry’s consumption of the products of other industries, as well as each industry’s contribution to the production process. With such a table it is possible to trace the effects of changes in one industry or sector upon all the other industries and sectors.

The usefulness of input–output analysis for forecasting purposes has been limited by a number of factors. One problem is that it takes years to put such complex sets of statistics together; in a changing economy, relations may have shifted by the time the data for a base period have been assembled. Progress has been made, however, in developing methods to bring these relations (called technical coefficients) up to date, and input–output analysis shows increasing promise as a forecasting tool.