A new paper, “Completeness and overlap in open access systems: Search engines, aggregate institutional repositories and physics-related open sources”. It tests the coverage of papers published from 2001-2013 by Nobel Prize winners in Physics. ‘Not all that many papers, then…’ you might think, but apparently 6,094 items were searched for.

96.8%  Google Scholar
96.5%  Astrophysics Data System
91.6%  Microsoft Academic (current version)
48%  OpenDOAR
45.3%  OAIster