Check out Process Efficiency in Scrum – Why it Matters and How to Measure it by Jeff Sutherland. Here is an excerpt:
Update: See Process Efficiency – Adapting Flow to the Agile Improvement Effort the first of several papers to be published. By definition, Lean means that process efficiency is greater than 25%. The name Scrum derives from watching lean hardware teams. A new team can drive process efficiency…
The full article is available here.