How does one measure the value of R&D?

Measuring the value of research and development (R&D) is a difficult problem.  Specifically, how does one measure the value of R&D on multifactor productivity (MFP).   At first glance, one would expect the value of R&D to be proportional to R&D spending in a given year.  However, this approach ignores that knowledge from previous years’ R&D…