Can you even define productivity? On the surface it appears as easy as getting work done, which if only a single person is on the team that may be enough. But once you add a group of people together, mapping a teams productivity to a single metric that can be applied to each individual becomes much harder if not impossible.

Basketball has had this problem for a long time. The plus/minus metric was invented as an attempt to address all the things players do while on the floor, but do not directly relate to an easy to capture metric. It also tries to address players who put their own personal productivity over that of the team.