Frascati Manual 1993
Trouble-shooting occasionally brings out the need for further R&D, but more frequently it involves the detection of faults in equipment or processes and results in minor modifications of standard equipment and processes. It should not, therefore, be included in R&D. After a new product or process has been turned over to production units, there will still be technical problems to be solved, some of which may demand further R&D. Such "feedback" R&D should be included.
Organisation for Economic Co-operation and Development (OECD), "Main Definitions and Conventions for the Measurement of Research and Experimental Development (R&D). A Summary of the Frascati Manual 1993", OECD, Paris, 1994, p.14