(en) Moses Abramovitz (Phrase originale « Since we know little about the causes of productivity increase, the indicated importance of this element may be taken to be some sort of measure of our ignorance about the causes of economic growth in the United States and some sort of indication of where we need to concentrate our attention. »), « Resource and Output Trends in the US since 1870 », NBER - National Bureau of economic research, occasional paper no 52, , p. 11 (lire en ligne [PDF])
