"Along with the shift in focus to investigating the sources and nature of business cycles, aggregate analysis underwent a methodological revolution. Previously, empirical knowledge had been organized in the form of equations, as was also the case for the early rational expectations models. Muth (1960), in his pioneering work on rational expectations, did not break with this system-of-equations tradition. For that reason, his econometric program did not come to dominate. Instead, the program which has prevailed is the one that organizes empirical knowledge around preferences, technology, information structure, and policy rules or arrangements. Sargent (1981) has led the development of tools for inferring values of parameters characterizing these elements, given the behavior of the aggregate time series. As a result, aggregate economics is no longer a separate and entirely different field from the rest of economics; it now uses the same tools and empirical knowledge as other branches of economics, such as finance, growth theory, public finance, and international economics. With this development, measurements and quantitative findings in those other fields can be used to restrict models of business cycles and make our knowledge about the quantitative importance of cyclical disturbances more precise."