20. M. Ishaq Nadiri and Theofanis P. Mamuneas, Infrastructure and Public R amp;D Investments and the Growth of Factor Productivity in U. S. Manufacturing Industries, NBER Working Paper No. 4845, August 1994; Douglas Holtz-Eakin and Amy Ellen Schwartz, Infrastructure in a Structural Model of Growth, NBER Working Paper No. 4824, August 1994; Dean Baker and Todd Schafer, The Case for Public Investment (Washington, D. C.: Economic Policy Institute, 1995), pp. 4, 6, 9.