The productivity impact of R & D investment : a comparison between the EU and the US / Davide Castellani (University of Reading), Mariacristina Piva (Università Cattolica del Sacro Cuore, Piacenza), Torben Schubert (CIRCLE, Lund University and Fraunhofer ISI), Marco Vivarelli (Università Cattolica del Sacro Cuore, Milano, UNU-MERIT and IZA) ; IZA. Bonn : IZA, May 2016