  We present a new conceptual definition of 'productivity' for sustainably
developing research software. Existing definitions are flawed as they are
short-term biased, thus devaluing long-term impact, which we consider to be the
principal goal. Taking a long-term view of productivity helps fix that problem.
We view the outputs of the development process as knowledge and user
satisfaction. User satisfaction is used as a proxy for effective quality. The
explicit emphasis on all knowledge produced, rather than just the
operationalizable knowledge (code) implies that human-reusable knowledge, i.e.
documentation, should also be greatly valued when producing research software.
