In an ideal world a development team would spend 100% of its time working on planned stories during a sprint. In reality this may rarely be the case. Most teams are interrupted by some unplanned work during a sprint. May it be a critical life bug, a urgent research or some production issues. An interesting question is how to measure the impact of this unplanned work. How does it affect the team’s velocity? How much more could be done without these interruptions?
Tonio Lucca created a nice prezi on that, with some impressive math foo, wich I may warmly recommend:
Good option if this is really possible to measure. The other question is how far can you get if you get early to work (e.g. 7 o clock – then there is no interruption at all.