9Bart van Ark, "International Comparisons of Output and Productivity: Manufacturing Productivity Performance of Ten Countries from 1950 to 1990", Monograph Series No. 1, Groningen Growth and Development Centre, 1993. http://www.ggdc, net/workpap.
10Bart van Ark and Marcel Timmer. "PPPs and International Productivity Comparison: Bottlenecks and New Directions", Joint World Bank-OECD seminar on purchasing power parities. Washington D. C.30 January-2 February 2001.