The findings we have summarized are consistent with a substantial body of research. For example, these findings coincide with those of L. R. Christensen (Wisconsin) and Jorgenson (1969, 1970, 1973) for the United States for the period 1929-1969. Angus Maddison (1987) (Groningen) gives similar results for six industrialized countries, including the United States, for the period 1913-1984. Assar Lindbeck (1983) (Stockholm) generally concurs. However, these findings contrast sharply with those of Abramovitz, Kendrick, and Solow, who emphasize productivity as the predominant growth source. At this point it is useful to describe the steps required to go from these earlier findings to the results we have summarized.

The first step is to decompose the contributions of capital and labor inputs into the separate contributions of capital and labor quality and the contributions of capital stock and hours worked. Capital stock and hours worked are a natural focus for input measurement, since capital input would be proportional to capital stock if capital inputs were homogeneous, while labor input would be proportional to hours worked if labor inputs were homogeneous. In fact, capital and labor inputs are enormously heterogeneous, so that measurement of these inputs requires detailed data on the components of each input. The growth rate of each input is a weighted average of the growth rates of its components. Weights are given by the shares of the components in the value of the input.

The development of measures of labor input reflecting heterogeneity is one of the many pathbreaking contributions by Denison to the analysis of sources of economic growth. The results presented in Table 2 are based on the work of Jorgenson, Frank M. Gollop (Boston College), and Barbara M. Fraumeni (Northeastern University) (1987). They have disaggregated labor input among 1,600 categories at the aggregate level, cross-classified by age, sex, education, class of employment, and occupation. These data on labor input have incorporated all the annual detail on employment, weeks, hours worked, and labor compensation published by the Bureau of the Census in the decennial "Census of Population" and the "Current Population Survey."

Our measures of capital input involve weighting of components of capital input by rental prices. Assets are cross-classified by age of the asset, class of asset, and legal form of organization. Different ages are weighted in accordance with profiles of relative efficiency constructed by Charles R. Hulten (University of Maryland) and Frank Wykoff (Pomona College) (Hulten and Wykoff, 1981; Hulten et al., 1989; Wykoff, 1989). An average of 3,535 components of capital input are distinguished at the aggregate level. Similarly, the data on