Book section: We emphasize an important point in the transformation of sources into data that affects economists and historians alike in every era: the theoretical framework they share. On a broad scale, three major moments can be distinguished during the twentieth century in terms of the general conceptual framework for economic data, which we shall call the price era, the quantity era and the individual era. In the first, economic activity is observed primarily through prices, which is justified by the preponderance of price theory. In the second, the organized measurement of quantities is produced by national accounting systems within the framework of Keynesian macroeconomic theory, and then of growth theory. Criticism of these earlier frameworks is now leading to experimentation with a very large number of measures, characterized by the sheer volume of information analyzed, greater vagueness between sources and data, and a multiplicity of conceptual frameworks. While each of these three stages represents a break with the previous ones, they nonetheless retain much of what has already been achieved.