Although the reliability of this theory as a predictive tool cannot be absolute, analyses of actual sources of cost reduction (as in the IEA Technology Roadmaps publications) show that its foundations are solid (Box 1). For example, Nemet (2006) studied the cost reductions of electricity from crystalline silicon PV modules from 1975 to 2001, and sought to disaggregate historic cost reductions - of factor 20 during this period - into observable technical factors. He identified three major factors of cost reductions from 1980 to 2001: manufacturing plant size, module efficiency and silicon cost. From its academic origin (Arrow, 1962), learning-by-doing is sometimes seen in the very narrow sense of "increased workers' productivity" due to experience, other factors remaining constant.