1.The level of productivity differs quite a lot among countries, which serves a major driver of income differences (China, 13% of US for now, and 40% of US in 20 years) 2.Productivity growth differs even more among countries, with average of 1.33% per year for 1975-209 among the fast-growing one-fifth of countries, and -1.42% in the lowest-growing one-fifth countries; 3.Overall, productivity growth explains 68% of the variation in income growth and factor accumulation explains only 32% across countries. 4.How productivity is determined? TFP “a measure of our ignorance?”