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)