In the course of the ensuing discussion, it became obvious that we're really using velocity to measure two entirely different things:
- the rate at which the team was completing useful work (and by extension, the rate at which we could plan future work), and
- the rate at which we were making progress toward getting the system we're building into production.
We decided to start tracking two velocity numbers by dividing our story cards into two categories: system features and everything else. We have a blue star stamp that we use to identify the system features, and we keep track of our velocity both overall and in "blue star stories." We've been tracking the fraction of our total effort that goes into completing those stories, and managed to raise it from around 60% into the upper 80s.
The obvious danger is that useful technical work will be thrown out along the way, but we've been reasonably good about not doing that.